Nuget,
Your basic description is of a motorhome. They all suffer when not used. Drive it and enjoy it. If it has a choke butterfly (which I doubt - they went out of fashion in the late 80s), spray that and the mechanism with some choke cleaner.
Yes, replace the water heater. If it is more than 10 years old, get the brake and cooling systems flushed. If some one tells you it isn't necessary, just shrug, say "OK" and get it done soon anyway.
Tires are only good for about 7 years. Got to and find out how to read the dates. Pushing this in hot places is very dangerous.
Before too long come back here with pictures and the manufacture and model of the coach and chassis.
Matt