The DOT does not care if its registered under 26k. They do care what the gvwr of the truck is. And most FL70's spec'd for expediting had 33,000 gvwr, the air brakes are usually a sign that tells that too. I looked at many of theses a couple years ago to build a truck to haul a Jeep but in the end scraped the idea.
They don't care if it is reg. as an RV. That's why you see 80 year olds driving 40 foot 50,000 lb motor homes around.