When I look up 7x12 cargo trailers on Google, they all have 2 axles and carry about 7000 lbs total (hence the trailer brakes). Empty they are 2000 lbs. Add the conversion and living stuff will add 1000 to maybe a high of 1500 lbs for a total of 3500 lbs. That puts a small vehicle completely out of the question. A Mini won’t pull 3500 lbs, period. Trailer Life magazine has a pretty good guide for this; http://www.trailerlife.com/trailer-towing-guides/
Pick the year vehicle you want and read up on towing and trailering. Look through the vehicles offered and the max tow ratings listed in the far right column and pick something that can tow 3500 lbs OR MORE (more is safer) and pick the vehicle that can tow it. Be sure any vehicle you buy really has the correct equipment - trailer or tow package, hitch, coolers, whatever the manufacturer says you MUST have to tow this weight. Don’t let the salesman sell you something he has on the lot the HE tells you will work (they are pretty ignorant about towing), have him prove it. Once you buy it, take your trailer to an empty lot and practice driving it and backing it up - a lot. Good luck.
I have a used police cruiser that I do light towing with. While it may handle the weight with appropriate upgrades (trailer brakes, air bags for weight handling, trailer hitch, etc) I would never do it. I only tow a small utility trailer with it, and I can tell it’s there when the trailer is empty.
I second the thought that a 1/2 ton or better pickup or SUV is required. Don’t even consider something smaller. If you’re going far, I’d upgrade the mirrors, add trailer brakes, and if you’re not good at backing it up get a backup camera. Diesel isn’t required, but would probably be nice.
I just reread the original post. Vision problems, op states that they do better with smaller vehicles and wants good gas mileage. Does not say if planning to live in the converted RV. My personal feeling is that you do not need to be driving a full size pickup with 12 feet of unbraked trailer behind you.
Every six-eight weeks is what stands out for me. Is this your home, and your job takes you to the other state regularly? How long do you spend in Ohio (assuming you live in Delaware)? I’m just asking these questions because very few people have this need. It’s not cheap or simple towing this trailer back and forth. If you have a permanent home in Delaware and only need the trailer when you’rlpe in Ohio you’d probably be better off leaving it in RV storage in Ohio and then you only have to drive yourself back and forth. If you find a conveniently located campground/RV park with long term rates you can just leave it there and drive your Mini back and forth. No truck towing that trailer is going to have decent gas mileage, unlike your Mini. If you have to put the trailer into an RV storage facility you’I’ll still need a tow vehicle. I’m sure you’ve considered the possibilities, but almost no one feels it necessary to make this kind of trip so often. They make other arrangements as a tow vehicle and fuel aren’t cheap.