I tow a jeep on a flat bed trailer that weighs close to what your rv weighs, but the shape is slightly different. On the open interstate I get 13.5 mpg and through back road highways I get 12.5mpg. I only drive 65 mph unless I burst to pass someone. I have a 2500 ctd.
On my first big travel trailer I had a tire blow out on the way back from a trip...I am pretty sure it was related to driving 75mph for several hundred miles to the park and about 100 miles heading home. The tires were old at that point as I had picked the trailer up used. The damage it did to the trailer was pretty bad and took a bunch of work to repair it. Now I stick to 65mph. I have not had a chance to pull my travel trailer (27' and about 8k lbs wet), so I don't know what I'll see with it. My old gasser used to get about 9mpg pulling both trailers, so I anticipate I'll see the same with the travel trailer.