I have ideas that there may be some lobbying involved. Rental companies, moving, hauling companies all could benefit from having less privately owned trailers kicking around.
Re road taxes. Unfortunately the big tax generators (States like CA and NY), and individuals with assets (homes, cars, trailers, etc.) are easy to levy fees that go into a fund to be disbursed by legislature. Fortunately only general funds can be spent wantonly. Fees generated by gas tax and license fees are earmarked by legislature for specific things. BUT, the govt has found a way to creatively ply "and other related uses" written into most laws. Therefore Busses, Trains, Parking lots, trails, bike lanes, landscaping etc sap $ that should be maintaining streets. I've very nearly time the populace of the u.s. do as our forefathers and lobby for users of facilities to be charged an appropriate fee to offset the operating cost. And, no more. Subsidies need to end. Too many of the working/middle class are expected to shoulder most og the countrys needs while welling populations at either end of the economic spectrum pay little or no to enjoy their lifestyles.