Regardless of the speed rating on the tires, I would not tow faster than 65 mph. You need to have extra time to slow and stop all that weight back there. No need to be in a hurry - you're on vacation! Exceeding 65 mph occasionally to pass is not a problem even if the tires are only rated at 65 mph.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)