Since you asked about a Paid source, RV Trader would probably be the best choice.
With that said try Craigslist first, as the Members above recommended. I sold or X19H quickly by only listing it on
our local Craigslist site yet the guy who bought it was 200 miles away.
I just sold 2 boats off of it (1 was picked up yesterday) and he came over 400 miles from SC and I only posted on 2 local Craigslist sites. All those deals were Cash including or X19H which was a 5 Digit Cash Sale.
There are sites online and Apps (I use one) that allow you to search nationwide very easily for items.
Key is a good listing header with words people would search for, (Brand, Year, Model, Length) lots of pics and a good description in the add. You can list on multiple Craigslist sites to get better exposure if like for people who may not use the nationwide searches.
You may get a couple scammer hits but you'll know who they are cause when you politely respond you will get no answer.
Good Luck with the Sale.
'16 28BHBE / Elite / Thermal / Dual AC / 50 Amp / 12K Equalizer
'15 F250 Lariat CC FX4 6.2 w/ 3.73 / Air Lift 5000's / Bilstein 4600 HD's / Bilstein 5100 Steering Stabilzer / Hellwig Sway Bar / ARE Z Cap
'12 X19H Hybrid (Sold)