Watch any consignment sale contract. I sold a Class A through a dealer consignment and it turned out to be less than satisfactory for me. The dealer was turning down offers I would have accepted, if I had been notified of the offer. Make sure you are involved in setting the asking price, I wasn't. Put everything in writing, I didn't. Also specify how long the dealer has to pay you or your bank if it sells.
Check the web site for PPLmotorhomes.com in Houston Texas. They are the largest consignment dealer in the US. Their home page gives lots of information on the process they use. They seem to have a good reputation with both buyers and sellers. They sell something like 2,000 RVs a year on consignment. The information they provide would be well worth reading IMHO.