Depending on how it was maintained I would agree 2000 to 3000 hours. Ive seen flat head propane powered fork lift engines with 6,000. Cleaner fuel, strict maintenance program.
An Onan could be around 2000 hours. A generac would be looking at less. A genset with very few hours could have gasoline sitting in carb and turning to varnish.
A gas engine one... couple thousand hours. A diesel... 8-10,000 hours would start to worry me but more because of the amount of use of the MH, not the generator.
Most problems with generators are due to lack of use and it's rare that a generator dies before the RV. Opinions vary but many exercise their generators a couple of hours per month just to ward off issues like gummed up carburetors etc.