I had good luck with rvtraderonline. Craigslist is free and allows pictures.
I would hold myself to a high standard on the quality of the pictures. Many rv pics are not flattering (specifically dark, out of focus, or poor composition), including ones posted on dealer ads. Zoom to wide angle for interior shots, use a camera which responds well to low light, and verify the picture is sharp, well exposed, and attractive. If not beg, borrow a camera which will take the kind of pics which are flattering to the rv. Make the picture as large as the venue allows within reason. Make sure the RV is shiny on the outside and clean on inside with shades open for viewing.
Lowball and middlemen come with the territory. A good presentation should help you get decent offer from private party buyer.