We also use a Thetford Curve and really like it (love would be a strong word for a porta potty)

But it's great.
But back to the tank discussion; Our 7x14 has axles in a position pretty far aft for my liking. If I'm not careful with the build, I'm worried that we'll end up with too high of a tongue weight. My observation of other trailers on this forum is that most of them look tongue-heavy, so maybe its not a big deal. But I'm going for maximum towability so my goal is a 15% tongue weight.
To help keep the tongue weight manageable I've taken an unusual approach and put my fresh and grey water tanks slightly behind the rear axle. The fresh water tank is 33 gallons and grey tank is 12 gallons. This equals about 270 lbs with the fresh tank full and grey empty. So I think this will balance the load and lighten up the tongue a bit without it being too light. How do I know? Well the weight is about the same weight as my dog and I combined. We all sleep in the back of the trailer with no appreciable change in the tongue height when its hooked to the truck....I had my wife check it.
I'm too lazy to weigh everything and measure the positions of everything in the trailer to do the actual calculations to give me 10-15% tongue weight. But I think my guess work will work out fine.
Anyone else have rear tanks?