I would thing any good regulator with a tee on the input side would do what you want. Just close the valve on the MT tank before open the full.
My Dad liked the auto change, but had a issue with his old camper. When the valve switched, the fridge would go out sometimes. He decided that while the tank was off being filled air would get in the line. When air bubble got to fridge, food warms. He put a tee in just ahead of regulator and a small valve to outside. Hook up fresh filled tank, close valve on other, open the new, then the little valve for just a second or 2. Open valve on the other tank, make sure drawing from the right one...
