Re: New Skoolie with a shortie (HELP!)
First, you say your bus "needs" batteries. Do you mean it needs STARTING batteries right now, or do you mean building a house battery bank is your first priority? Don't try to use deep cycle batteries to start your diesel. They might work sometimes, but I expect you need a lot of punch to turn over the 6.2. We had four of these engines in military surplus pickups, and even with two batteries wired for 24 volt starters we used jumper cables A LOT.
For a house battery bank, I think marine deep-cycle batteries are often a cross between starting and deep cycle constructions. Golf cart batteries are true deep-cycle batteries, as are the ones made for solar systems, etc. As I understand it, starting batteries have many thin plates to provide high-current shots for a few seconds. Deep-cycle batteries have fewer, thicker plates to provide low current for a long time.
What you get for batteries depends on your budget. Top dollar for hydrates would be Rolls or Trojan batteries. Marine batteries from a big-box club store are a convenient and less expensive option, and many travelers appreciate the fact that they can find a nearby store for warranty replacement wherever they go. On the cheap, try to scrounge used 6-volt golf cart batteries, match a set in similar condition, and nurse them along.
If you use hydrates, you should ventilate the battery box(es), and check the water from time to time. If you pony up for sealed AGM batteries, you can carry them inside the bus without too many concerns, just watch the voltages.
The choice of how many batteries and what voltage you wire them for to is up to you. If you have a 12-volt alternator, a natural choice would be to use 12 volts for the bank voltage, so you can charge them through an isolator, switch, or relay while you drive. There are lots of 12 volt automotive and RV devices to run off of them, also. The 12 volts could be stored in one or more 12-volt batteries in parallel, or one or more series pairs of 6-volt batteries. Avoid mixing and matching, your best performance will be with all the batteries the same.
If you will only be using all 120-volt AC devices through an inverter, higher voltages might be a desirable choice. There are inverters for off-grid use that work on 12, 24, 48, and other voltages. The higher the voltage, the less current is needed to provide the same wattage, so less power is wasted heating the wires, and smaller wires can be used. But that means no charging off of the engine, and more solar panels to build up to the voltage. The panels must have a slightly higher voltage (pressure) than the batteries in order to force current back into them.
You should definitely plan on using an MPPT solar charge controller, and having the solar array at a higher voltage than the battery bank. For example, use panels wired for 28 or 32 volts to charge a 12-volt bank. Direct wire of cells to the batteries, with controller that only shuts down at full charge, causes the solar panel voltage to be pulled down to battery voltage. This reduces the wattage, so the result is that the more your batteries need a charge, the less the solar will supply. Ask me how I know. Also, panels are made of many individual cells. If part of a panel gets shadowed, the voltage will drop. If the voltage was much higher to begin with, the MPPT can adjust for losing some of that overhead and continue charging.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.