Depending on the RV owners manual you consult, and I've consulted quite a few, there are five basic systems in most recreational vehicles. These core RV systems are:
1. RV Electrical System
2. Entertainment System
3. Fuel & Propane System
4. Heating & Cooling System
5. Plumbing System
6. Slideout System
When I was living in my Jayco 5th wheel RV on a full-time basis, my heating and cooling system worked very well. I was warm and toasty in the winter, and cool in the summer. By the way, as far as I know, the rooftop air conditioner and furnace were both original.
I'm not really sure what the average operational life span of RV air conditioners or furnaces is, but I would imagine that original units installed in a seventeen year old travel trailer would be getting toward the end of their useful lives.