As the owner of a 2000 American coach, the SINGLE MOST IMPORTANT item to check is done by laying on your back and inspecting the underneath of the basement floor. The stupid engineers at Fleetwood build the basement compartments from steel tubing, and then covered it with aluminum sheet. The aluminum reacts with the steel and corrodes away, exposing the steel tubing to road water. Then the steel tubing rusts away and before you know it, the basement is falling off of the coach.
We have had our coach for over a decade and still enjoy it very much. The "Heart" inverter/charger is very outdated if it is still being used. The engine is a Cummins ISC and will last pretty much forever However, that engine has a known weakness with the design of the exhaust manifold leaking from the gasket and may need to be replaced.
The design of the windshield support is weak and tends to cause broken windshields unless you learn very quickly how to extend the leveling jacks properly. AC decided to use three jacks rather than 4. Procedure is to raise the single front jack first, than level side to side with the rear jacks while pivoting on the single front jacks.
The chassis is a Spartan Mountain Master which is well built and solid. Historically, three biggest problems are front ball joints, the rear height control rod failure, and (the absolute worst!!) the power steering/engine cooling system. There is a hydraulic pump on the engine which sends hydraulic fluid from a 5 gallon reservoir to a hydraulic motor which drives the side mount cooling fan, and also sends hydraulic fluid to the power steering system way up front. This system requires periodic maintenance and if not done, fails in a very big way. Failure means you can't steer the coach, and failure also means your engine overheats. The overheating sets off a very loud beeping alarm that will wake the dead, and also shuts down the engine, no matter where you are. Neither of these situations are fun to handle.
An annoying feature of the coach design are the basement door "slam latches" which are difficult to get to latch properly and will be a constant source of close attention prior to leaving on a trip. If you fail to verify all latches are latched, you may have a basement door fly open while driving which can lead to some serious problems as you can imagine.
Having said all this, we still enjoy ours, and after putting on about 140,000 miles, I would not hesitate to get in it and head across country for an extended trip. It is very comfortable to travel in and to live in.