I've never heard of mice nesting in a generator exhaust pipe. I think the Onans have (or at least are supposed to have) a spark arrestor screen at the end of the exhaust pipe anyhow, which would prevent any mice from getting further in than that. Likewise vehicle exhaust systems in general are not especially problematic from that point of view, at least that I've heard of. Doubtless it can occur and has happened once or twice somewhere, but not at anything like a probability that's worth losing any sleep over.
There's not a lot that a cold exhaust pipe has that mice like: it's not warm, it has no bedding material, and there's nothing to eat. Nests in the generator itself (with the air cleaner etc.) or of course in the RV house part generally can be problematic. Removing all traces of foodstuffs and not heating the interior do go a good distance towards avoiding problems (but not eliminating them altogether).
