I believe these type of "sighting" categories was one of the original ideas that completely flopped since its really only for the purpose of setting up a waymark and the likelihood of someone visiting it in the same exact place is rare. And I don't think they are prevalent/unique enough to warrant a category like the UHaul Supergraphics.
It could work really well under the Photo Goal category though. "Take a picture of a vehicle used specifically for marketing a product" i.e. the Truly Nolen mouse car, the wienermobile, etc. I've seen some weird-looking vehicles for restaurants and they use them in parades or new store openings. Obviously it needs to be more than just a paint job or those car wraps, there needs to be some major design changes to the vehicle to qualify.
Another photo goal spin-off could be spotting a celebrity RV/trailer on the road or at a stadium. It could apply to NASCAR, musicians, sports teams, etc.