Lady,
You have a pile of info above on which to cogitate. The plusses in your case are:
1. 2500 truck which will require little upgrading if you get a light weight camper.
2. gas engine- increases your load rating without the diesel boat anchor which subtracts about 5-600 pounds from the payload rating.
3. If it's only you in the box, an 8'6" camper is plenty enough. You don't even need a N/S bed. Add another person or a couple small dogs and suddenly it isn't.
That being said, the rule of thumb is, "the longer you are out on the road in your camper, increasingly, the more space you will need", unless you are real good at space utilization. Following the warmth is another issue. If you go south for the winter and north for the summer, you may not need air conditioning. Depends on how tough you are and how tuned to the seasons. Do you have any hobbies you will take with you? Space in a TC is at an absolute premium. There are a few small campers that are 4-season. the full featured Lance 650 is one for a S.B.
One thing to think about are your knees. There is an ever increasing effort to get up the steps and up the two big steps to the bed. Be sure to try that out several times in a row. A 2WD truck will help in getting up the steps: not so high off the ground.
I'm the same age as yourself and don't take quite the chances I did when I was 40, so it would be good to iron out all the details to make sure a TC is a good fit, and do some close trial runs before you set sail. You may find leaving most of the stuff you took with you in previous RV's is liberating, especially while traveling/parking/overnighting. Good luck to you, and do let this forum know how it works out.
jefe