A tag or registration that is expired in the U.S. is only a problem in the U.S. Mexican officials only are interested in the proper importation of the vehicle, in the case of a tourist with a vehicle it is for 180 days for a car or truck and a 10 year permit for an RV, a person entering on a tourist visa is issued a 180 day visa same as the car.
The expired tag will be a problem while transiting through what ever border town in the States you happen to cross back, every cop in every town will issue a ticket, the best thing to do is to pay the registration and have the documents stating that the registration has been paid on the vehicle and letting the police know that you are going to pick them up real quick or have family or friends send the tags via UPS.
navegator