I have a 30ft. Itasca, I tow my wrangler which they say has a curb weight of 3760 to 4315 lbs. I have aftermarket bumpers and such so I would say it is on the heavier side.
I have a ready brake system and towing the Jeep I show no difference in fuel consumption. I went to Vegas 275 each way and got 7 to 8 mpg. I towed my Jeep to Vegas once
and got 7 to 8 mpg. Towed a 20ft. trailer weighing in at 5000lbs and got 5 mpg, same trip, coming from Vegas to Orange Ca. Flat towing has no effect on my motorhome that I
can feel other than minimal fuel consumption. BTW, when towingI rarely go over 65 mph. California is 55 speed limit, I stretch that to 62 using my cruise control. love it.