If you are going to live in it then that is a whole different story than just a vacationer.
Yes, go big and then some. Keep empty spaces as you will need room to do stuff inside at times, so have option to move stuff around, this is a given if it is a toy hauler-style.
You cannot just flip axles. Axles have an arch so have to stay upright, but you can mount the leaf springs on top of the axle and gain some inches, or just have a straight axle put on to a drop axle design and this will also give height. I replaced a 4 inch drop axle with a straight and then mounted springs on top and gained probably 10 inches or so.
As for V-nose verses flat verses bull-nose. The V is not going to be aerodynamic like it looks like it would be. The V is a sharp angle and will not move the air smoothly and will not do well with cross winds. You report the same as other V owners, no mpg gain over a flat.
Now, a Bull Nose trailer has a curved front and WILL increase mpg's as the air moving around the sides is passing much more smoothly.
The curved bull nose is harder to build out inside, but is worth it.
If you are having the trailer built for you then consider a slightly curved roof too. This will move water, snow and ice off better and will give you more head-room in the center where you stand without having to be tall all the way to the sides where you do not stand. I have to park my trailer nose up to move water off of it so it does not collect anywhere.
I am talking about a slight curve as maybe 3 inches difference from side to center. The solar will still work just as if it were flat.
I think the U-haul trailers are built like this, among others.
There is a guy on youtube that lives in his and it is a cargo trailer of a massive scale, wide, tall and long. He has under-floor storage, and a split level in the back for bedroom and a sitting (not standing) room.
It will take a lot of time and effort if you are a perfectionist and do not want to use 2x4's and pre-fab homedepot cabinets and waste space by being a sloppy builder. You will use over 1,000 screws, and do not use nails, screwed-and-glued is my motto.
I hope you can weld, using steel will save space and make life so much better.
good luck