Aged fuel may limit performance under load but you should still have a reasonable idle. Try the generator. If it runs with a high electrical load then old gasoline is not the problem.
Fuel filter is a bit of a long shot but an easy an inexpensive fix. A partially clogged filter will allow a good idle but the engine will stumble when a higher rate of fuel is needed such as acceleration or a high climb. A sudden fully clogged filter can happen but is a bit rare as compared to a partial blockage occurring first.
I'm thinking you have a dead fuel pump or fuel pressure regulator. Easy to test with a pressure test kit you get as loaner for an auto parts store. Not sure what your pressures should be for throttle body injection 460 V8 but I would expect to see nothing less than may 30 PSI at the injector feed.