Diesel, gas, or propane generator?
Most problems with generators end up being a fuel problem. Note that a '99 gas or diesel would probably not run if the RV's fuel tank is below around 1/4 tank as that's were they get fuel so if your tank is low, that could be the cause. The other things that happen is the fuel pump goes bad, or the filter is plugged and the big one in an older rig is that the rubber hose has rotted out and the generator is drawing air.
And of course those engines need to be serviced regularly, especially the gas models.