It's very normal to not be able to be in the transmissions highest gear. And also normal to run at higher rpms when doing so. Think about it, your vehicle has just been asked to pull a combined weight of the vehicle AND the R-Pod. Definitely get an auxiliary transmission cooler, it will keep your transmission fluid cooler (burned-up, overheated fluid is what kills transmissions). Don't bother with highest gear, leave your auto transmission in the next to highest gear, unless climbing hills, then I recommend slowing down to keep the engine from getting near redline.
If you can give up the minivan (Honda Odyssey) and go to an SUV, see if you can afford a used Touareg TDI V6... You will get the best of two worlds, a very strong tow vehicle, and very good mileage when not towing. Also add that it handles like a sedan, not a truck. I'm biased on this because I have one and won't go back to a gas powered vehicle. Someday i plan on getting a bigger trailer (small toy hauler) in the 5000-6000 pound range and I will still be well within the tow rating (7700 pounds).
If Jeep works through their issues with the new diesel (mostly electrical problems from what I read), then a used Grand Cherokee diesel would work good as well. It was on my very short list of SUV's when I was deciding what to buy.