People tell me they think I should get rid of all my old vehicles. I'm not willing to do so because each of them has a fully functional, rechargeable 12 volt system with around 48 amp hours each, which means that, fully charged, they will each deliver 1 amp for 48 hours, 2 amps for 24 hours, 8 amps for 6 hours and so on. Each can provide light, heat, shelter, and storage. I install inexpensive solar on the rooftops which provides light, charging of electronics, and available power for my tools. I further use them for safe storage of equipment and utilize them as greenhouse space when frost threatens. An old van with a lot of windows is best for that.
Think about it. Each unit collects power, stores power, and can generate power if more is needed. People will even pay you to store their vehicles and start them up once in a while to keep them alive. Look at what you have (or could have) available to you before you consider buying anything.
In such a system, each vehicle is actually a separate circuit. It doesn't matter at all if the batteries are the same or not because they are isolated from each other. Even in a normal house with conventional wiring, different zones or areas are usually governed by separate circuits. This allows for smaller (and safer) fuses and breakers. In my solar home, each outlet with low power demands gets its own isolated solar array, battery, and charge controller, which completely avoids the problem of matching batteries, but there are some loads that that require more current or relatively lower current for longer durations, and this is where multiple batteries on one circuit comes into play.
Even if you buy batteries from the same vendor, manufactured in the same batch, at the same time, and all of them have been exposed to the same environment with the same charging conditions, there will be differences between them. It's best to match them with partners which are as nearly identical as possible.
To do this, you measure the OCV (open current voltage) after they have been resting for at least 24 hours. You can sometimes do this in the store before purchasing if the vendor has a large inventory, but make sure you only try to match those with the same dates. The vendor will prefer you do this rather than to buy ten times more than you need with the intention of returning 90% of them after matching. Another approach is to purchase larger batches with others who also need batteries, match them, and then divy them up.
Once your batteries are grouped, they should stay together as a team. Charge them together, discharge them together, and store them together. They should then be treated are a single unit. If you later decide to increase the size of a bank of batteries, don't even think about adding batteries by buying from a different vendor, or even from the same vendor for that matter. For instance, if you have three matched batteries and you want to increase to a five battery bank, then remove the three for use on a lesser circuit and start with five freshly matched batteries.
The reason this is important is that unmatched batteries in parallel will fight with each other. You're forcing them to always have the exact same voltage and they might not want to do that. Think about it like workers who don't work well with each other because they are of a different mind with different intentions. In series, it's much less of a concern. For instance, you can use two six volt batteries in series to produce 12 volts, and they won't fight with each other the way two twelve volt batteries in parallel will. It's because each of the two six volters will be allowed to have a different voltage level than the other.