I recently bought a barely used '06 4x4 RB50 SD for just over $80k and ran into the same problem. Your typical lender will book out the van only, and have no idea how to assign a value to the conversion. Even GE, who finances new vehicles for SMB, assigned a maximum loan amount on mine of about $40k! This is a vehicle that's less than a year old, that they would finance new for the full value of $100k. Go figure. I talked to several RV lenders with the same results.
The best thing to do is find a good lender, and then it's really a negotiation on the value. A broker that has access and leverage with several vendors is also a good way to go and can save you a ton of frustration.
I finally ended up with a recommendation to a broker that secures financing for high-end boats. He'd done a Sportsmobile before, and negotiated a value with the lender based on the blue book, plus a list of other used values that I sent him, mainly from listings on the SMB site. They valued the vehicle at $79k, and I put about 20% down, at 7%. Not too bad. (They would have financed up to the full $79k). The loan ended up with regions Bank, which may also be a good starting point. Contact me if you want more detail, and I can dig up the name of the broker. Good luck.
Truckee, Lake Tahoe, CA
'06 E350 Diesel, Quigley 4x4, RB-50