Everything depends on where you are going & how long. If you stay in the Free Zone of Sonora & Baja, you need nothing except Mexican Insurance. You need at a min Liability per Mexican Law. I use Lewis & Lewis purchased directly off the Internet. You may wish to call them and ask about the Trooper. I have not thus far, I have it listed under my Pickup's policy but I don't know if it would hold up as it says it is pulled, like a trailer.
After 3 days in the Free Zone, you are required to have a Mexican Visa.
Outside of the Free Zone, Mexican Ins., Visa & Vehicle Permit, ALWAYS.
All this is detailed in a Sticky on rv.net under the Traveling in Mexico Forum. RV.Net Open Roads Forum: New and Improved Border FAQ's
rocmoc n AZ/Mexico