FWIW
There is a Performance Data table in TM 9-2320-260-10 on page 1-26 with the following maximum speeds listed:
60MPH, M812A1, M821.
52MPH, M813, M813A1, M814, M815, M816, M817, M818, M819, M820, M820A1, M820A2
The only vehicles with a reduced maximum speed listing for "w/towed load" are the M816 and M819 where it is dropped to 25MPH!!
This assumes an "as issued" truck...
I personally have a hard time running faster than the TM specified limits just because I hate to break things, and I'd hate to try to expain to my insurance agent why I was running faster than the "design" limit...