As far as a full tank adding to your pin, it really depends on where the tank is. Granted, mine is a bumper pull, but the freshwater tank is directly over the axles and does nothing to the tongue empty or full. If your tank is close to or over your axles, it won't change your pin weight noticeably. That said, if you dry camp and your grey/black are forward of your axles, you'll be shifting a lot of water and could end up with a heavier pin leaving than you came in with. Just dump as soon as you can in that case.
Because axles are so far back on toy most toy haulers, loading things close to the front tends to have a greater effect on the pin weight than on non-toy haulers. Move some of that load to the garage if you want to reduce your pin weight. With only a couple of atvs, you should have plenty of room to pack in beverages (I take lots of extra water with me) and gear around them. The only hassle is moving it once you set up to camp. Also, if your fuel tank is behind your axles, fill that before you go.
Remember, though, it's not a direct 1 to 1 weight gain of stuff in the garage to weight loss on the pin. It's more like a 5 to 1 or even 10 to 1 dependent upon your axle placement.
Good luck! Glad you are enjoying your hauler.
I'm done. This isn't a place to be helpful. It's a place where curmudgeons with a superiority complex will nit pick everything. If you want help, go elsewhere. Admin, delete my account please.