I'm in the camp with those that say replace it with an original unit if you intend to turn a profit on it. I can't imagine anyone being interested paying retail price for the the motorhome knowing someone had removed the generator. It would definitely be a red flag and beg the questions "what else has been removed, and why?
If you just want to recover your money I wouldn't replace the generator.
I would guess you got an exception deal on the motorhome primarily because of the missing generator and the fact that the bank didn't want to deal with replacing it only to find there could be other serious problems or missing components. I'm also guessing the next buyer would be looking at the same pricing structure for the same reasons.
Yesterday I Googled "Onan 5500 RV generator" and came up with at least a dozen hits for used units. They ranged in price from $1,900.00 to $4,600.00. There do seem to be several available.
Personally I would check local or regional authorized Onan repair facilities. There's always the chance someone had one repaired and either chose to replace it instead or traded for a different size or model.
Our local Onan repair station had a couple refurbished units in stock the last time I was there. That was a while ago, and I didn't ask the prices because I wasn't in the market.
Hikerdogs
2013 Winnebago Adventurer