Parking BIG RV in Mexico?

Yes, but Mexican trucks are trucks and aren't crossing borders, or if they do it is as cargo carriers. US cars come in on a TIP subject to certain restrictions - ie weight. RVs are a different classification that requires a certain type of registered class. Presumably proper US trucks drive in on a completely different system. Pickups aren't any of those types so there is no form of entry for them so they are refused.

Of course the figures under which they are being refused are being misinterpreted, but sorting that ouyt might take months