I hate to spend more of your money, but most likely you just should replace the original Univolt (assuming it is original) since they are known to be hard on batteries, and your new ones will suffer the same fate.
On replacing the batteries you have, assuming you have a real mess of broken battery and plates and parts, first disconnect the univolt from the system by unplugging it or turning it off at the CB, or turning the entire trailer off by unplugging it from power. Then disconnect the Un-exploaded battery and remove it. Clean the area around it (I don't know how your batteries are mounted, or located, are they in two different compartments?). Do not put the new one in yet.
The exploded battery needs far more care. (rubber gloves and goggles for eye protection of course) Have a lot of baking soda available to neutralize the acid, I would guess you need several pounds (boxes). Pour it liberally over the remains of the battery. It will foam a lot. Add water if you can drain the mess that results out without getting into the remainder of the trailer. More baking soda, more water until you have most of the mess neutralized. Then remove the remains of the battery. If possible dispose of the results into a plastic container and take it to a recycling center. It should not go into a dumpster. More baking soda, and water until your battery location is all neutralized. Then dry it and put the new batteries in place.
I would NOT leave the univolt on any more than absolutely necessary until it is checked out, or replaced.
That is an outline only, just be very careful please.