In most RVs the generator is given priority at the transfer switch, but if the generator isn't producing output, the transfer won't know it is there and would use shore power instead. So, odds are you are actually on shore power.
It could be the transfer switch, but check out the generator first as Jeff suggested. You probably have the Onan QD 8000 and that has a breaker on the generator itself, Turn it off and back on again and see if that fixes the problem.
If that's not it, the next step is to test the voltage at the generator wires to the transfer switch using a voltmeter, but do not do that yourself unless familiar with live 120v wiring. It has to be tested with live voltage and could be fatal if you make a mistake.