Exactly this. Its gonna be real hard to finance one from someone who built it.Depends on the risk to the lender. If you are talking a mainstream manufactured Sprinter RV that lends credibility. If you are buying a backyard one off Sprinter Van Camper Conversion, they will be unlikely to value it above the bare Sprinter Van value. They might even regard a home built conversion as too high a risk. For those risks to the lender you will need a specialist who will likely charge a premium.
A main stream manufactured Sprinter Camper Van, don't call it a conversion, will have traceable value and almost any lender will be able to assess value from the used market.