Start the rotors, lol!
There are a handful of guys that have done similar on this Forum. I am one. There are many folks out there who try this but never complete it. The pictured restoration of my Starcraft is linked in my sig line--"The Cowboy/Hilton".
The really great thing about a scratch build is that the owner can rearrange everything to his liking and improve upon what came out of the factory.
Suggestions--
Keep the aluminum siding and the aluminum roof (if you have one). Those materials have been used for 80 years and are superior. You'll have to decide on initial process but I did mine from the inside-->out. Weather and a few key advantages ruled that. Plan out all the mechanicals/electric and draw it all out with fixtures located. Your CAD skills will shine here but a guy could do it with a cocktail napkin. The trick is to allow for locations keeping in mind the schedule of installation. Ie, You don't want to find that your new load center cover is impossible to open because there is a partition or other fixture in the way. I forgot one switch box location and the separate location and circuit for the 120V fridge. A panel removal and a router solved that quickly (I got lucky, the fridge location is only 4' from the load center).
Further suggestions--Insulation, a guy can't pack enough of the best available in a TT. Your Prowler will probably have a 1 1/2"-> 1 3/4" stud depth. The best is construction grade spray foam. I used 1/2" spun fiberglass and 1" extruded polystyrene, each piece custom cut to friction fit the cavities (floor & ceiling too). Over the stud wall interior is a panel of 4' x 1/4" extruded polystyrene and a vapor barrier.
Windows, I removed a few and replaced some with Low-E sliders. The front window is a Low-E stationary panel.
The combination of windows and insulation results in a very well insulated box and the sound level has to be experienced--it is tomb-like quiet inside.
One of the biggest head-scratchers was relocating the toilet. I was married to the waste tank location so a new drain and vent needed to be installed taking into consideration the tank location. The shower stall never left the trailer as it couldn't be removed (that's OK, I like it where it is).
Good luck on the scratch build. Suggest you weigh the alternatives of first procedures (from out->in or in->out). BTW, budget was discussed in the Hilton thread but it's sort of buried. I had some materials/appliances on hand but construction costs ended up to be around $4K. I have since, added solar charging and more batteries--another $1k.
Marc Polk RV Garage trailer rebuild series on You Tube You'll get sick because Marc gets a lot of help from advertisers/sponsors but it's all there.
