No restriction on trailer towing speeds here out in Arizona and I believe most adjacent states are similar. Posted limits are generally 75 mph (except Utah and Texas which have some higher ones) and most of these states will give you 10 mph grace.

For those going to Region 9 in Moriarty, I can confirm that speeding fines in New Mexico are very affordable (I got $75 for a trailer-free 92 mph).

Mike