What you are learning is how much work it takes to pull any high profile trailer through the air at 60-65mph. That's why all gas engines pulling all high walled travel trailers get 8-10mpg. Your Amerilite is probably narrower and shorter than most travel trailers, thus the 11mpg :). Pulling with a V8 Tahoe would also get very close to the same 11mpg, but more comfortably I expect.
The V6 in your minivan makes good power, probably at least 250hp and 250ft-lb or torque. However, it does not make a lot extra at it's normal operating range, say under 2500rpm. Plenty for the van and your family, but not the reserve for another 3000 pounds and a lot more wind resistance. So when you need more power, it will rev, probably spending a lot more time in the 2500-4000rpm range. That uses the extra fuel too.
If you want to lower engine speeds, then you'll need something that makes more power where you want it to be. A V8 in a Tahoe for example will make more power at every engine speed, thus being able to tow at a little lower engine speeds. Today's modern turbocharged gas or diesel engines will do even better, providing up to double the amount of torque at 2000rpm you currently have.
Do you need 3 rows of seats? If yes, then go straight to the long wheelbase which for GM is the Suburban. That 3rd row is a penalty box in the Tahoe combined with only carrying a few grocery bags behind that. If you only need 2 rows of seats, I highly recommend a crew cab pickup. New they cost about $8-10k less than a comparably equipped SUV. They offer the worlds largest truck if you add some kind of bed cover. Their payloads and tow ratings are higher. Ford's 2.7L Ecoboost in the 2015 F-150 would be great for you.
If you want the least amount of fuel used, a diesel would be it. Fiat Chrysler America offers a good one in the Jeep Grand Cherokee as well as Ram 1500. The European imports also have some, MB, BMW, VW/Audi. All would get closer to 15mpg towing your RV and do so never exceeding 2500rpm. Statistically, diesel fuel costs more per gallon in most areas of the country over the last several years and the engine choice costs more to purchase up front than a gas engine. You'll have to decide if that would be valuable.
A & A parents of DD 2005, DS1 2007, DS2 2009
2011 Suburban 2500 6.0L 3.73 pulling 2011 Heartland North Trail 28BRS
2017 Subaru Outback 3.6R
2x 2023 Chevrolet Bolt EUV (Gray and Black Twins)