Question I have:
- how to replace bathroom floor - are the top and bottom attached in such a way that the floor cannot be replaced?
Definitely doable, Google search airforums shell on floor replacement. It's a little complicated since you need access to the bolts so you'll have to remove some interior wall panels.
- what is the best insulation to use that nice won't eat and stops water and doesn't pollute or poison us
For that I'd recommend rock wool, it's hydrophobic and pests won't eat it or live in it, it's hard to find some the right thickness so it can take more work.
- cad program my wife can use to design new interior
Depends on how detailed she's going to get. For exact dimensional layouts I'll second the google sketch up, there are online tutorials. For just general layout ideas something like PowerPoint can be pretty easy.
- do I really need vents for grey water ? I have bought a compost toilet so do not need vents for grey water tank - do the sinks really need it? I would love to not have to install all the pipes from floor to ceiling like there used to be.
Yes you need to vent the tank. Going through the roof isn't that bad. You can route the plumbing to minimize intrusion. If you don't vent thevtank you can get the suction effect mentioned above, but as the tank fills up pressure would build up and stop water from draining into it, significantly reducing your effective tank capacity.
- suggested new wiring for a new build.
I'll keep my input simple. Think and plan in great detail. How you plan to use the trailer. Always hooked up, boondocking? What do you want for entertainment equipment. Radio, tv, speakers, cables and outlets. Inverter and converter upgrades, solar? I inspected, repaired, and reused most of my 12v
wiring, but added a few new circuits and upgraded the fuse panels. I replaced all my 120v, adding several new circuits and more outlets for convenience.