When it comes to the tires - I will leave that to those who are much better versed than I.
As to tongue weight, I don't see the angle impacting the weight significantly. The only way I could see there being a real difference is if you carry partially full tanks. It is conceivable that a nose-down configuration would cause the fluid to rest forward in the tank (bringing the weight forward) while the nose-up would cause it to rest in the rear of the tank, bringing the weight back - resulting in a small change in tongue weight.
With that said, while this is theoretically possible, I also imagine that the real impact would be relatively negligible. The impact of fluids on tongue weight depends heavily on the location of the tank, but shifting 150 pounds of weight 2 feet further back is not going to have an impact worth consideration.