IF you're doing the work yourself, a Versatube structure is pretty easy to build and cheap. $6000 or so for a 24x24' and it goes up in a weekend with a couple helpers. That is of course just the steel itself, no pad, electric, plumbing, etc.
What will the insulation needs be? Climate? A pre-engineered shed from the parking lot of Lowes or Home Depot might work. Add some R6 batting and drywall.
I have been an RVer all my life including 6 years as a full timer, so I can steer you in the right direction on brands/years/etc, but always remember that no matter how amazingly built a camper is, it is always engineered for the average camper.... they'll use it on Memorial day, 4th of July, and Labor day. 2 weeks a year, tops. Single best, hands down, most wonderful camper I have ever owned was a 92 Holiday Rambler 32 FK. I bought it in 2003 after the previous owners lived in it for 4 months a year, now I used it for 16 years for 2 months a year. I never covered it, and it has endured Ontario winters completely exposed. I just sold it this summer. I never had a leak, the solid oak cabinets never had any sagging doors, the upholstery looked brand new. The only thing that I did was rip out the carpet and put down linoleum because it finally started showing wear. I did have to repair the fridge ($70 circuit board), but despite a total of nearly 30 years of abuse, I had trouble getting rid of it, it was that nice. It was a wonderful trailer. All aluminum flat skin, aluminum studs, and "real" cabinets.
Contrast that with mom and dad's brand new 2018 Cruise Lite. They used it for 3 weeks this summer and two of the particle board cabinet doors fell off, the rest of them the hinges are such cheap brass that they are all bent and the doors sag and won't close, the electric awning switch died, the cheapy vinyl upholstery on the couch is already pulling the seams apart, and the LED lighting on the patio quit working. It is cardboard and paint. IMO, newer RVs are just appallingly horrific. The layouts are awful, the materials are absolute crap, and they have more shinola kitchy baubles that all seem to fail immediately. I go to the Hershey RV show every year. Last year I looked at what I thought was the best-made trailer on the lot (A Forrest River), and the pocket door for the bedroom had already fallen off during delivery. I just watched a promo video for a new Prowler 39' luxury trailer and I LOL'd the whole way through. $43,000 for stick frame, corrugated aluminum sides, and a rubber roof.
RVs are also a bit cramped. In order to get two double beds, you're looking at a park model of 35-40' at least. Some of them have a front and rear bedroom. But no RV will have a bedroom with two beds unless they are twin bunks. If you're comparing a 1.5 car garage (which would be about 18 x 24'), you have double the square footage as even the largest RV.
Of all of your options above, I would still personally buy an older RV and spit polish it, but I think that would be the worst possible scenario for you. If you can spare the space in the shop, build in a bedroom. 2x4s and drywall. Done. Super cheap, super easy. 20 years on an RV isn't out of the question - especially in a garage, but it will be dark, cramped, and skinny. Not really a "luxury" option. Its only real benefit is that it can be moved if needed.
The shop already exists, it's already plumbed, you're building additional garage space in the future, make a room or two in the shop. It's already under a roof, so construction would be minimal. If they're in their 60s now, those three steps will only get harder to navigate, but a bedroom in the shop has no steps.
I often think about tiny homes on a trailer, but they often represent the maximum investment in design, engineering, materials cost. I would rather take an existing, well-built travel trailer or 5th wheel and modify it.
Another suggestion not on your list: Buy a $2000 decommissioned tractor trailer van and stuff real cabinets and appliances in it. Buy a gallon of white elastomeric roof goop and make sure its dry. That is a recipe for an inexpensive palette on which to paint a tiny home.