Given everything you've listed here, I would suggest a travel trailer + truck combo. Something with a sofa, dinette, bedroom with a door (helpful for couples) and a slideout for extra space so you are not cramped with each other. The slideout allows 2 people to move around at the same time without bumping into each other. If you were alone you could leave the slide in if you want to. The bedroom with a door allows you to sleep late or nap without disturbing each other so much.
You might also look at the rear living type floor plan, very nice layout for a couple.
A trailer is less expensive than a 5th wheel or motorhome, and as you said, allows you to NOT maintain 2 motorized vehicles.
IMHO the most useful tow vehicle is a full-size SUV. The exact one you need is dependent on the trailer weight/length trailer you choose. A SUV gives you extra secure storage space that is more accessible than a truck bed + topper. Buy it right, and if you decide you don't like it, you can sell it without too much of a depreciation hit.
Whatever you get, add in a good weight-distributing hitch for the combo. Worth every penny.
I agree with the advice to rent somewhere before you buy. Take the camper out and stay in some different areas and get a feel for them. Go back to the ones that you like the most and investigate how they would be to live in for 6 months at a time. Investigate things like access to medical care, taxes, COL, general amenities.
I love the area around Big Timber, MT. Beautiful, nice people, nice small towns. That would be one of my first places to investigate for long-term summer living.