Re: how long will they last
Hi Kathi-27,
I don't think there's any specific life expectancy on furnaces and water heaters. They require a little bit of periodic maintenance, but should work for an indefinite period. If they do fail, it's usually a component that needs to be replaced. It's rare to have to replace the entire unit.
Air conditioners are a little different. Although they can last a very long time, if they fail, you're probably going to have the entire unit replaced. RV techs aren't trained to do much in the way of air conditioner repair.
There's a school of thought that appliances may last longer in full time, rather than seasonal use. It's pretty common to see posts in the Spring that start by saying "it ran fine when I put the rig into storage for the winter".