As a general rule, we never tow with anything in the tanks....very minor exceptions excluded. Water weighs something like 8.3# per gallon x 30 gallos, if that's your tank size = 240# in the FW. Probably similar in the grey and I haven't asked and likely won't how much the "stuff" in the black tank weighs per gallon. But adding 750 - 800# without realizing it is easy. I couldn't help but think as I hauled the WM bags to Rocky the other day just how heavy they were. I really should park the bathroom scale by the TT and just see how much weight I'm adding just in food, etc for a weekend trip. Probably would knock my socks off.
Slightly OT, but my cousin had his own plane and 3 small children. As the children grew, so did the weight and the "stuff" that came along. It got to the point that his DW "joked" - not really - that when they found they had to weigh the potato chips, it was time to sell that plane. Same principal.
We are very weight concious, but when we started moving stuff from our HTT to our new TT, we were amazed at the stuff that had found its way in there. Likely 1 piece at a time, but it added up. Rocky is on a lifetime diet! If it's not necessary and/or we don't use it regularly, it stays home.