I haven't towed a Jeep, but I have towed a toad over 11,000 miles in my Interstate.
You may want to look into getting a RennTech ECU tune for the Sprinter. It will improve horsepower AND fuel economy at speeds of 60mph and lower, but if you go faster than 60mph, fuel economy will drop like a stone. If you get it, get used to poking along at 60mph whether you're towing or not. The engine will also run about 5° hotter, hardly enough to tell the difference on the temperature gauge in the message center.
My toad has a curb weight of 2700 pounds and a GVWR of 3500 pounds. It's routinely loaded to about 3100 pounds when I'm towing because I use my toad as overflow storage while in transit. With the RennTech ECU tune I routinely get over 18mpg on flat terrain near sea level while towing at 60mph. Crossing the continental divide in July at 7280 feet in New Mexico, I was still getting over 15mpg while towing at 60mph. That's an improvement of about 2mpg versus before getting the ECU tune. The horsepower difference is mostly noticed when accelerating at a highway on-ramp, which is really the only time you need to pour on the coal while towing.
One caveat… make sure that when you set up your Interstate and Jeep that the towbar is level. I can't count the number of people who pull a toad and ignore this rule. If the towbar has a rise or drop so that it's pulling at an upward or downward angle you won't get the best possible fuel economy because the van will be constantly trying to lift or push down the front end of the toad. The proper setup has the towbar level so that the van's power is all going to a straight horizontal pull, with no up or down component.
So if the Jeep's bumper is higher than the Interstate's receiver, you'll want a kit from Blue Ox, which normally has the attachment point on the toad below the bumper. If the Jeep's bumper is level with the Interstate's receiver, you'll want a Roadmaster kit because Roadmaster typically positions the attachment points on the toad level with the bumper. The chance that the Interstate's receiver will be higher than the toad's bumper is so remote that there's no point considering it.
I don't know enough about the Stowaway towbars to really talk about them, except to note that the Stowaway fold up on top of or right in front of the toad's bumper, and attaches to a regular hitch ball on the van. The Roadmaster and Blue Ox systems typically have a towbar mounted on the van's receiver, and attach to the baseplates on the toad.
Here's a picture of my Roadmaster setup…