The problem with open market covers is that they are made to be easy to put on and take off with several different top choices (ie hard or soft factory, bikini, bimini etc). This makes them poor choices for towing as they will flap and chaff simply because they are not tight and do not control air flow THROUGH the covered vehicle...
I had a tow cover made that goes around the cowl a full 4 inches ahead of the windshield (also covers the grate in the cowl to stop air from going into the cabin through there) and all the way down the sill line. The bottom cinches in 6 places down the side, sealing to the splash panels and the back is vented. This causes the air pressure to push the cover to the metal - while the venting allows the LOW air pressure inside (caused by the vacuum behind the vehicle while moving) to seal the sides. No flapping - but must be put on in a specific manner so throwing it on while on trail and caught in a downpour is not as easy as the besttop for example. Doable but I am always the last of the group to be covered - by twice as long as the next to last! Fair trade off though... mines the only one I have seen that tows without flapping.
The guy that made it for me specializes in boat towing covers... and just followed the same logic there - seal the front against air entry, keep the sides tight over the top and back vented and let the air do the work - and took about a week to custom cut all the pieces to hit all the exact points needed to accomplish. Not cheap though at 950 - but the fabric is super lightweight and marine grade for high UV protection. When trailered at 70 the cover looks like its vacuum wrapped to the Jeep - no flapping or chaffing! Not sure about how well it would do in the vacuum behind an RV though... my trailer DOES have a big box on front - but its no taller than the hood so the windshield is still up in a very high pressure area.
J Wm Bishop EA, ASADE
Alcohol - because great stories never run on gasoline...