Someone has already mentioned how full-timing will likely create cargo issues.  The other thing will be the size of your 3 water tanks (fresh, gray, black).  If you are staying inside National Parks most are dry-camping arrangements and you'll be relying on your tanks rather than hook-ups.
You'll likely find any TT or fiver built for full-timing will max out your payload.  And you could quickly get tired of the upkeep needed with a "vacation" unit that is getting 100% use.
Although slides add terrific space to a TT, they also can make it very hard to use with the slide pulled in (for things as simple as accessing the bathroom).  I feel we got real lucky and our floorplan works great for lunch stops and such.
Another consideration is seating.  Our dinette and sofa in the TT are great for a weekend or a bit longer, but I would be upgrading both if we were going to live in it.
I'd say for what your goal is a 1-ton truck with a short fiver is going to be more realistic.