Still looking, need tall interior, but half ton towable

You might look at a smaller 5th wheel. Ignore the "1/2 ton tow-able" marketing and look at the ratings (in particular the pin weight when loaded).

You probably won't get full stand up headroom in the bedroom but in the main living area, you should get upwards of 7' headroom.

The other thing to consider is what is the trade in value vs what you can buy on a heavier duty truck. If you can upgrade the truck to a 3/4 ton a whole lot of options open up in terms of what you can tow.