When was the last time the heads were off to de-carbon and the valves were adjusted? That would be a decent place to start. Recommendation on valves and cleaning I think is 200 hours. Onan parts has a gasket kit just for this job which is handy.
Yep, Boomer says some of those cam bearings start peeling apart in layers. He won’t touch anything in the engine without doing the cam bearings.From past experience if the welch plug is loose it was caused by the cam bearings being worn and the cam itself moving around. Something else to check when you have it apart