I have pulled half a dozen enclosed trailers in the past 30 years. Pulled on over 100,000 miles behind our old Class C.
Our last one, we put over 50,000 miles on it behind our 31 foot Class A.
The 7x12 tandam axle trailer we pulled behind our Georgie Boy with a Ford 460 didnt change its Milage at all. It got 10MPG
towing it and got 10MPG not towing... I think it was the gearing on it.. That old one didnt have overdrive and the motor was turning
some pretty good RPM's at 70MPH... But that motorhome could accelerate with any traffic ! It also never slowed down going over the mountains..
It would go over them at 55-60MPH with no problem...Towing a 12 foot enclosed trailer !
Our last MH, a Coachmen 31 Foot Class A with the Chevy 454 towed a 7x12 single axle enclosed trailer all over the US. We got 7.5 to 8.5 MPG.
We took one trip to S.C. with out the trailer and we got 7.8 to 8.7 MPG each tank full.... Slightly better not towing.
This MH had overdrive, but it was a bigger heavier MH 18,000lbs compared to 12,000 on our old class C
Our currant MH is a Thor Outlaw Toyhauler, so we dont need to pull a trailer anymore, But we did have to pull our trailer back home from La where we traded out old MH in.. We got 7.7 MPG on the 1,000 mile trip home. The Outlaw has the Ford v-10 motor in it with a 5 speed tranny. This motor runs higher RPM, and the Outlaw weighs more, loaded 24,000lbs, and it is taller, (13.3 foot tall) so I dont think we will get as good a MPG average as our old one... We are getting ready for a trip to Florida soon so we will find out..
As far as towing, sway etc. A trailer will affect you slightly, When a semi pass's you it will push the trailer slightly which you have to steer to correct for it.. How much depends on how much rear overhang on the MH.. The longer the tail, the more a trailer will affect it. On our Old Class C which was short with a very short overhang, the tandom axle trailer hardly affected it at all..... Now the Outlaw with a Long overhang pulling the single axle trailer is affected much more... A couple times on our return home trip, we got passed by a semi and and we got pushed around a bunch. My wife thought something was wrong ! I told her, "its because of the longer MH and rear overhang" . Keep it below 80 MPH!.. (she likes to drive faster than me)
Bob