Re: 1996 Newmar Mountain Aire 40'
Since it works on the generator and not on the house, the problem is almost certainly in your transfer switch (or whatever your unit uses for this purpose). In some units, this is an automatic box which chooses which power source to use. These have been known to stick. Find it and give it a whack, see what happens. It is probably an aluminum box about 1 foot long and 3 inches by 6 inches. The generator and the house line will feed it and the output will go to the circuit breaker box.
I suppose there might be a manual transfer switch where you manually select your source of power. I've never seen one in a RV, but I have one in my house. In this case there would be a switch on the box or perhaps remotely, hopefully labeled with the 2 choices.
The final option is no transfer switch. In this case, you have to move the plug which powers the RV to either the line cord or the generator.
Your best bet is to read the manual if available, or contact the manufacturer. Other than that, all you can do is trace power wires and try to find where the selection of power occurs (since it would be a Bad Thing to have both power sources connected together)