Since my 1996 Blue Bird TC2000 already came with an 8D battery, I bought 2 more Deka 8D batteries that put out 225 amp hours each. I got these for $125 each on Ebay which is a steal because normally they go for $400. I wired in a 1000 amp surge cut-off switch that was $15 from Harbor Freight with 1/0 gauge cable between the starting battery positive and the 2 house batteries positive since there may be times when I don't want to take a chance of my main battery dying. (forget to bring generator, jumper cables or battery charger on a camping/hunting/tailgate trip) but for the most part, I will be using all 3 together at the same time. I'm not really worried about an "equal load" drain right now but when I hook up a total of 4 or more batteries, I will have to hook up the ground wire at the last battery in the circuit. This way, each battery gets depleted equally. If you don't believe me, check out the link Smitty mentioned earlier above: http://www.smartgauge.co.uk/batt_con.html
I'm not really worried about an isolator yet either.
I upgraded all my batteries positive and ground wires to 1/0 gauge from the stock 4 gauge, my alternator positive to starter battery wire and also my engine block ground to chassis frame with 1/0 gauge. This is called "the big 3" and is cheap and well worth it. I called DCPowerINC.com and they hooked me up with a new Delco 250 amp alternator that has a compatible J-180 mount and puts out around 150 amps @ idle for $350. This is a big difference from my stock 1996 Leece Neville that only put out 130 max and around 50 amps at idle. One of the main reasons for me wanting a larger alternator was due to having a TV, lights, 3000 watts worth of stereo amplifiers and of course, the 2 new batteries.
As for power inversion, I have a 5000 watt/10,000 watt max modified sine wave inverter with 2 gauge wire and ring terminals connected to the 3rd battery. There are 4 outlets on this inverter and I plan on drawing <1,500 watts and <20 amps from each one to be safe. I then simply ran an extension cord and power strip to wherever I wanted on the bus. (1 in the back, 2 on each side and 1 up front) Since the 8D batteries I bought were wet cell and not AGM sealed, I had to keep them under the bus due to fumes. Angle iron was welded to the frame along with a sheet metal box. I can still access the terminals easily for charging/disconnecting. I made sure that all batteries were charged to 12.6 volts before I hooked them up. My Victron Energy BMV 600 monitor shows a voltage of 14.4 while the bus is running. It also shows how many amp/hours is still available. If the bus sits for more than 2 months at a time, I'll throw a trickle charger on them. I thought about solar panels during the day for trickle, but since I already have a generator and battery charger I decided to wait for now.
To determine if your alternator is strong enough to power everything, watch this video:
You will want to do the math and add up all of your appliance's wattage and amperage loads. This will determine your power inverter size, amp/hours battery bank size and how long it will all run before you need to turn on your bus, charge the batteries or hook up the generator.
Wattage = Volts x Amps
Amps = Wattage / Volts
AC Watts / 12 volts x 1.11 = DC Amps (for an inverter operating @ 90% efficiency)