Just curious as to why you need US insurance? Plan on some road trips stateside?
I know for me to get Mexican insurance through Vagabundos (which insures oodles of RVs, boats, trailers, etc.) I HAVE TO have a valid US policy. Then, they offer two types - 1) a Tourist policy (mostly for the free zone areas and Baja) stay not to exceed 180 days and 2) full-year anywhere in Mexico insurance (more for the ex-pats).
I do, however, agree that if it the coverage about Mexico is not specifically written in the policy, then make NO assumption that you have it. That's one of the reasons I insure my boat through a small agency in Redlands, CA. Because the owners are fishermen, they specifically add a "South Coast" endorsement to the policy, which covers operation of the boat throughout the Pacific and Sea of Cortez to a distance not more than 50 miles south of Cabo San Lucas. It's a nice add-on at no extra cost.
I guess what I'm getting at is that if you do some shopping with different agencies, rather than corporate insurance, you may find a policy that has more features and coverage than you expected. Go over to the Vags forum and see what the Mexico RVers there commonly use and why.
Just my .02
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