We used to sell at county fairs and locations like Quartzsite. Normally we would park our 5th wheel at a nearby campground/RV park and drive to the event each day to work our booth (EZ-Up Awning and tables with merchandise). Our 5th wheel is 38Ft so it was very comfortable for end of day relaxing.
At the events where we could park the 5th wheel at the site where we sold from (like Quartzsite and some of the fairs) We never had a problem getting into the site but would have to make sure we were able to get a large enough site to fit in. That meant paying more rent for the site than our usual 10X20 ft site we used at most venues where we couldn't have the RV.
We already had the 5th wheel before we began selling so we didn't have a choice of an RV that was more conducive to our needs. Knowing what I do now, I would get a 25-30 Ft bumper pull toyhauler. And tow it with a full size van or suburban type vehicle. Or a pickup with a cap. The toyhauler provides room to carry all the stuff you need to setup your booth, supplies and whatever you are selling. The garage area of the toyhauler can be built up to fit your needs. The enclosed van, suburban & pickup with a cap allows additional cargo space protected from prying eyes and the elements.
A 25-30Ft trailer should be small enough to allow you to park in the space you are assigned at venues where you can have the RV at your sales space.