Finding used Northstars is like finding a unicorn. We searched for months without success which is why we have a 2019 we bought new. It is almost impossible to lose on a Northstar. One time we were on our way to pick one up and it was sold out from under us when we were a 100 miles away.
As to them being tall, yeh, I find they are tall and top heavy. It almost invariably means the addition of some aftermarket components to improve handling.
On the models we looked at in the past like the one in the picture the boxes detach and when you back under it wraps around the rear fenders of the truck.
Where does your gray water tank dump hose connect, out the back wall or underneath. For the gray, all you need is a regular garden hose to dump so if you can connect one, do nothing at all.