I was contacted by a similar site - Once Driven - and the problem is they are legitimate (legal) but in my opinion not worth the fee they charge.
I went to their website and found a listing of a Cl A MH- then went to Google and did a search for that make / model / yr and many units for sale popped up but
none from the website selling their service.
Here's a very good post re: how to sell your RV - I agree with many of the suggestions and have done almost exactly what was suggested...several leads and folks looking - hasn't sold yet but confident it will...
Biggest issue is to price it right - high prices do turn off some folks - my feeling is a priced to sell price and holding firm if necessary will get the most traffic the soonest.