If it has an automatic it more than likely has an SV345.
To help with drivablity you may want to upgrade to a fuel injection system. It might increase your fuel mileage a little but the main thing it will do is make starting a lot easier, it will run much smoother at any elevation or humidity, and it won't be nearly as tempermental getting going.
My friend Bill Hamilton has developed a plug and play system that even those who know next to nothing about any engine can make the system work.
Hamilton Fuel Injection
The Allison AT545 is a great light/medium duty transmission. It does not lock up in any gear so be careful the first few time you go down steep grades so it doesn't get away from you.
If you have 20" tires and wheels now they are most likely OEM wheels. 20" would be tube type tires. If that is the case definitely opt to go with the 22" wheels which would be tubeless. Depending upon where you are and who you know it may actually be cheaper to purchase tubeless wheels and tires for less than it would cost for a set of tubes and tube type tires.
At a guess they are 8.25X20 or 9.00X20 tires. Those are going to be hard to find. At one time they were a very common size of tire. But with the advent of good tubeless tires in the early '80's those sizes have become very old fashioned. Old fashioned=lots of $$$$.
10R22.5 would be an almost direct replacement for the 9.00X20 tires. You might be able to squeeze a set of 11R22.5 tires which would be a direct replacement for 10.00X20 tires. The 11R tires are about an inch taller and will give you about 3-5 MPH more at the same RPM.
The 11R tires might actually be the least expensive option since it is still a very popular tire size. The other sizes because they are not very popular may require you to have to purchase premium brand tires with premium $$$$ prices.
Don't be afraid of purchasing off brand cheap Chinese knockoffs if you purchase them for a reputable tire store. Your tires will die of old age long before you ever wear them out so there is no reason to spend the extra $$$ for premium name brand tires.
About the floor plan--lay everything out with painter's tape on the floor and cardboard boxes for any of the fixtures. Then take the bus out camping and use the way in which you hope to use it. You will determine very quickly what works and what doesn't work in real life compared to a CAD on your computer.
Good luck and happy trails!