BINGO!When I figured it out it was simple when I got rid of those Mickey Mouse breakers and put a real breaker box on top.
Glad you said that because. Most people dont know that.The generator is actually two separate generators in one core. That's why it runs at 3,600 rpm's instead of 1,800 like others. It is possible for one side to go out and the other still work.
SkateBoard, I finally got to look at my RM, problem was belt tensioner pulley had broken, belt was off, FOI, the pulley is a Gates item and can be obtained locally, kind of a PIA to install but finally got it done and making electricity, also put gasket under the air filter, didn't help much, went back and used duct-tape. Herb..The air filters are RM on mine and it still makes noise unless I tape it or make a gasket so the cover fits on tighter. You need to put a volt meter on the cab cord and see if there is 3 or 5 volts coming out. If you do get a reading chances are a new capacitor will fix it.
Also, you should not be plugging in your block heater. It's a big drain on you generator when the element gets old. If your batteries are low and the RM alternator is charging along with the block heater and other stuff it can take it's toll on the generator head.
If you have a smart phone you can download a sound meter app to see the effect of the air cleaner cover. Keep in mind, the sound meter is NOT a linear scale just like the earth quake scale. 100 is not twice 50.
Glad it was a simple fix!!!
Thought I might add: the part # for the idler pulley on the belt tensioner is : Gates #38001, this is also the # for the other pulley on the belt, they are common pulleys, most parts houses stock them. Herb..Glad it was a simple fix!!!