Why not call an Authorized Onan Service Center located "somewhere" (in the USA) near you? Tell the service department what service you require, the model generator you have, and the year, make, model of your motorhome. They won't be the cheapest, but at least you'll have some idea of what's in store for you.
Depending on where you are located, a qualified RV mobile service tech should be able to help you, but their price will depend on how far they have to travel to get to you. IMO, it's a good idea to take care of regularly scheduled maintenance (even if over due) now, before you are really out in the boonies.
Chum lee