Part of how I make a living is designing electrical systems for cruising boats. I've learned a lot over the years and lots of new components are availalbe.
I'll throw out a few tidbits...
For the best estimate of actual power draw always convert to watts first 'cause while amps and voltage change from system to system...watts is watts.
The voltage at which AC appliances are rated varies (some 115v, some 110v, some 120v, etc) and we're going to use a factor of 10 to convert that to our typical DC system so inacuracies get multiplied. If you figure watts on the AC side (amps of draw X the rated voltage) then you can divide by your DC system voltage to get actual amperage draw. It isn't typically 12-volts by the way; that only exists a one point in time. It's not unsual for the system voltage to be well over 13 volts when charging and quite a bit lower as power is consumed from the batteries.
Going back to our Watts = Amps times Voltage you need to know that Watts is the power the appliance uses and is a static number (that's not ture for things that have variable speeds or temperature settings but humor me and pretend for now; the max draw or various ratings are usually on the label) ; that makes it easy on us since that means the other two numbers (Amps and Volts) must always multiply together to equal Watts.
Lets do the math...if you have a refrigerator that draws 3 amps at 115 volts that's 345 watts. Now you know how much of your inverter's capabilites this thing will use. To figure the DC side there are two options (at least)...the first is to multiple the amps draw on the AC side by 10; in this case that would be 34 amps (which would be #6 wire if the inverter is 15' away from the batteries). We use 10 rather than 12 because it's an easy number to divide by and because it leaves a "fudge factor" for inverter efficiency and less than ideal wiring runs. The second (and more precise way if it makes a difference) is to divide the Watts you calculated by DC system voltage. A fully charged battery (the kind we're talking about) is 12.8 volts and a 50% discharged battery (the level you shouldn't go below and unless you have "traction" (deep-discharge) batteries) is 12.1 volt; so using 12-volts presents sort of a worst case scenario. The amperage in the DC side in our example will be 28.75 amps but then we have to figure in inverter efficiency (listed in most of the Owners Manuals). If it's 85% then that works out to 33.82 volts; awfully darn close to just using our "multiply by 10" method.
Unless you have a 3-stage charger (either an AC model you plug in at home or to a genset, or one on your bus alternator) it's difficult to charge deep cycle batteries up past around 85% of their capacity. On the flip side (unless we have special deep-discharge batteries) typical deep cycle batteries should not be dischaged below their 50% capacity level. For day in and day out use that means we typically have available about 35% of the batteries rated amp-hours.
Using the refrigerator above as an expample which is using 34 amps, that means if the refrigerator runs 50% of the time we'll consume 400 amp-hours over the course of a 24-hour day. We would need a battery bank of 1165 amp-hours to support it without damage to the batteries (that is, to not go below 12.1 volts). That's three 4D batteries or eleven group 31's!
A battery is (by definition) "dead" at 10.5 volts but plates in typical deep cycle batteries are physically damaged when they're discharged below 50% of their capacity (about 12.1 volts).
An inverter should have a battery bank to draw from that's at least 20% in amp hours of the inverter's rating in watts. That is, a 1000-watt inverter should be connected to a minimum 200 amp-hour battery bank.
A battery bank should not be larger than 3 times (ideally) to 4 times (ok) in amp-hours the alternator's rating in amps. That is, the 200 amp-hour bank above should be charged with a 50-amp alternator as a minimum and better yet with a 65-amp or higher alternator. [As an aside...the 1165 amp-hour battery bank we needed from above to run our refrigerator should be charged by 388-amp alternator!]
When calculating wire size remember the rating is always based on the *round trip* from the battery to the load; not just one way. They're also based on the wire being in "free air" and not bundled; if your wire travels through tight conduit and/or is bundled with a bunch of other wires step up a size.
Allow for voltage drop; we shoot for less than a 3% drop on a wiring run in boats for sensitive electronics, running lights (their output has to meet legal requirements), pumps and motors. Other circuits get along fine with a 10% maximum drop. There are wire charts available for both (West Marine has them on their web site http://www.westmarine.com/pdf/0660_ETRIC_MC04.pdf
If you locate your batteries far from your alternator make sure you size them for the engine's starting loads even if they're house batteries unless they're isolated (manual battery switch, battery combiner, etc). Also remember to use the round trip distance to calculate the size necessary to carry all the amperage the alternator will put out. For example...you'll need #4 wire for an alternator of 130-amps if the batteries are 15 feet away.
Just remember that we're talking about a SYSTEM; every component is interelated. With the room available in the typical bus is wouldn't be much of a stretch to get 500 amp-hours (or way more) of batteries installed; but then you have to charge them with something and if that something is an itsy-bitsy alternator on the engine you'll have to drive from Detroit to Dallas to charge the batteries if they're discharged much.
Lots of folks have systems way outside of the parameters I've outlined here. That doesn't mean they won't work; they just happen to work in specific circumstances. For instance if you use the example from earlier (1165 amp-hours of batteries and a 130-amp alteratior) it would work fine for someone who goes away for a weekend of camping. Let's say they drive 4 hours to where they're going, spend the night, the next day, and another night then drive home where they back the bus in and hook up the shore power cable and the battery charger goes to work until the next outing. Chances are with that setup they never took the batteries down all that far in the course of a couple of days and they really didn't need them recharged on the road (although the alternator did what it could) since they were heading home where the charger was available. If you take that same rig and keep it out longer with no shore power available and short road trips the batteries will just stair step down until they're dead since there's no practical way without an onboard AC charger connected to a power pole or a genset to recharge that large a battery bank with that small an alternator (unless the bus is always on the move).
There is absolutely no implication here that you have to do your system any particular way. I'm only posting this as "knowledge base" so to speak for folks that are interested in such things. It's your bus, do it your way but if you're interested in getting the most out of your system (and it does take a bit of learning and monitoring) there's some information here to start you out and it outlines the basics (even if you're just tring to purchase an portable inverter).