Tires/Water tank.

I took my TT out last weekend to mess around with WDH and to be honest I just wanted to.:) I have my TT and truck pretty much loaded with everything except I plan to carry water. My TT is closed in underneath so I cant see where tank is but I do see a large bulge in the cardboard covering up near the front. Can I assume that's where the fresh water tank is? I really wanted to keep my 285-16 tires on because I'm 57 and haven't grown up yet but I see that's not going to work because of soft sidewall and I think they make my truck allot more underpowered. I have some stock size Goodyear ultra grip LT- 245/75/16 Load range E that are like new .They are a mud and snow tire and I'm thinking a soft compound but I wanted to ask what you think about using them. Also, I no its best to have the TT level or nose down a little and that is my plan but wondering if nose up takes away tung weight? Thank you.