I had a very basic (previous rental) 2001 class C that did not have a transfer switch. To use the generator, the user had to plug the shore power cord in to a 30amp outlet in the shore cord compartment which was wired from the generator. This also prevented the condition of having both energized at the same time as TheLooks described.
During a rather complex solar install that would run my AC off battery/solar power, and involved a victron 3000/24 inverter, six residential solar panels and 2/3 of a Nissan Leaf Li-ion battery, I also installed a transfer switch.
But that’s what “sparked” my questions. If it worked before then something changed and the first place I’d look is the breaker on the generator as TheLooks suggested. Then I’d get out the multi-meter and start investigating.