I believe there's a listing of RV parks on the Business Chamber website. Not all RV Parks are set up for vendors. There's swap meets where you can park and sell on both sides of the freeway. Check out Rice Ranch, Quartzsite Marketplace, Tyson Wells Showgrounds, Vito's RV, Prospector's Panorama, Desert Gardens- all on Kuehn. It appears Greasewood Swap meet is rented for the season by Mr. Motorhome, but across the street is Hi Ally Swap Meet, with others on Main St: Main Event, Main St. Traders Swap Meet, Quartzsite Showgrounds, , and there's a new place opening on W. Main Call 360-961-3866 (Rush Signs).
Hope that helps