It is mostly a CYA thing the manufacturers put into their manuals to get out of paying warranty claims that get too expensive. There are also regulations and rules about permanent dwellings that would be expensive to comply with. Full time or commercial use is very taxing on RV systems and as a result full timers would be more likely to file more claims and for larger amounts. A good example is one of the largest RV manufacturers in the world installs "modified sine wave" inverters into their rigs with a residential refrigerator and microwave convection oven. They know that the refrigerator will fail after about 1 - 1.5 years if running all the time due to the poor power quality. They made the intentional choice to use this square wave inverter knowing that most people will use the RV for only a few weeks in a year the microwave and or refrigerator will last at least past the warranty and it won't be their problem. Engineering a coach for full time use is a very expensive prospect because everything needs to be made more sturdy and able to resist wear. There are a few manufacturers that do build coaches intended for commercial or full time use and they are very expensive and at the same time have more industrial components. They are also much heavier for a given size of RV. You will see these most commonly in the racing circuit. These coaches are 80,000# with the trailer.
