We drove a VW bus in Mexico that the registration tags expired half way trough the 180 day permit, my mother in law at the time paid for the tags and sent copies of the paper work, not the tags, no one said a word, for the insurance purposes you might go on line and pay for the tags, the actual tags may be sent to your rescidence or have them waiing for you at the first RV campground when you enter the U.S.
If you pay trough the internet, print all emails and all documents, you should have a paper that states that the tags have been paid.
If you mean the expiration on the actual vehicle importation permit, you better head for the border before the actual expiration, if it is the insurance, that you can extend any time
navegator