Thanks for the replies already.
wolfe10: I'll get the part and model number, all I know off the top is that it's an Onan Emerald 7000 in my 1994 Bounder.
enblethen: I'm hoping it's an adapter screwed into the block. With a flashlight yesterday it looked like aluminum threads and didn't look like an adapter. I'll remove the generator and take off all the shrouds, it's about time for an out-of-vehicle service anyway. I'll probably wait for next Spring: our winter weather arrived with a vengeance this morning and we're expecting at least 2 inches of rain a day for the next several days. After that it'll just be regular rain for the next 6 months.
We cant Wait: 'Dis-similar metals fusing together usually occurs only after a prolonged period of contact - like YEARS' I agree with you. I don't use the generator much but do change the oil/filter once a year. I've never seen anything like it.