In the wind speeds you mentioned, we would probably have slowed to about 55 as well. Generally speaking, we draw the line at winds over about 38 mph, especially if they're crosswinds. We'll just stay put when they get too high - it's not worth the white knuckles. As Scott pointed out, winds
can blow you over. For extra stability towing, as it's almost always windy where we have lived, we usually fill the freshwater tank at least 1/3 before leaving home - good ballast.
Yes, gas mileage will drop into the wind. Anything that's taller than your tow vehicle is a sail. No experience with 5ers, but I would guess some of the big tall ones would be even worse in that respect, though they might possibly handle a bit better in wind.
2020 Toyota Tundra CrewMax 5.7L V8 w/ tow pkg, Equal-i-zer
2020 Lance 2375