It is tricky question so don't expect straight answer.
As you figured already- you are to some degree in charge of COG placement.
You fill up water tank - it will move forward, you put case of beer under rear dinette - it will move back.
The biggest hit will be carrying waste water, so how far you can control that?
Seeing Northstar 650SC listed at 1600 lb, you can expect loaded weigh in the range of 2500lb, what either way will add between 2300 and 2700lb on rear axle. Probably way over 3000 lb if you carry waste water in tanks.
Start with finding a pickup who can handle that + whatever you intend to tow.