I used to teach scuba diving every other weekend in Mexico...Not sure what this Importation permit is, but in my experience I have never been asked or had it mentioned, I have no idea what it is. With all that said, I would never pull my RV down into Mexico. Some have with good luck and some with really bad luck. Remember, you don't have any "Rights" in Mexico, they offer nothing you can't find in the US and the prices are no longer a great deal. Their diesel fuel is high sulfur, so if running a newer truck, you will have problems.
My Ranch is 13 miles from the Mexican border and I use to go over for fuel for the ranch tractors etc. Coming back home about 1/4 mile from crossing into US, there was a backup at the border. About 6 cars behind me, It sounded like an automatic weapon firing....When at the border, I asked what happened and someone was wasted while in their vehicle crossing into the US from Mexico....
I have never been back since and will never ever go back again....In my opinion, stay in the US or Canada, Mexico offers you nothing that is worth the risk you take by crossing over....Also I had a Dr buddy placed in jail, seized his truck, boat at one of their interior check points run by 16 year old kids with automatic rifles. They said he has drugs in his boat...never happened and he lost everything they took, and spent over 6 months in a Mexican prison before being released by paying a lot of money.