Most of us with MHs usually cruise at between 60 and 65 even if the toad has no tow speed limit. We typically get the best milage at those speeds.
I tow a CRV and typicaly cruise at 62-64 and occasionally go above 70 on some down grades. As Sky Boss eluded, the limit is not a hard "never exceed". You simply don't want to tow above that speed for any length of time.
Paul, Kathy, and Tux a 4 month Mini Schnauzer
2014 Tiffin Phaeton 42 LH, 2013 Honda CRV
"When the time comes to look back, make sure you'll like what you see"