MPG: my diesel x5 got 14-16mpg towing an enclosed v-nose at nearly 9k lbs. pulling a flat nose enclosed at6k lbs I only get 12.5mph. So you are within reasonable on gas mileage.

BRAKING: those Uhaul trailers have surge brakes which may or may not be adjusted well. But at the end of the day you’ve doubled the weight of the moving package but not doubled th braking capacity x5 brakes are vastly better than the trailer brakes). You should expect/assume degraded braking performance when towing anything.

SPEED: And IIRC the trailer itself is not supposed to go over 55mph. This might be for your safety depending on what tires were on that trailer. Nice that you didn’t get pulled over for towing too fast for the trailer.