In Australia here - so can't help with finding a composite supplier.
Having built a few truck campers from composites, some of my tips - feel free to ignore...
On my "never again" list, painting the shell. It's messy, stinks, hard work, lots of prep, and probably toxic. Get a composite in any shade of white that you like.
Have a think about how you want to run roof wiring to a fan and lights. My composite guy glues conduit inside the composite panel where I need to run wires. Otherwise you can put in a false ceiling or simply go the industrial look.
Buy pre-made hatches and doors, find a good supplier or get them from Ebay/Amazon. Cheap hatches will still work better than your first few attempts and save you weeks of work.
Fully design, and preferably build the interior in dummy cardboard (or even final build) before ordering the shell. The hatch locations might change the shell, the water fill location might change the shell etc. I actually fully build the interior, run wiring and plumbing, then drop it inside my shell.
Also, I order parts the other way around now. I order the doors/windows/hatches before the shell. That way if the manufacturer no longer makes certain dimension components, they'll still fit my shell, and match my interior.
For example - I know where my internal water tank needs to go - then I know where my water fill needs to go - then I know what size hatch can fit around the water fill - then I know what cutouts I need in my shell
The hardest part is trying to get it right the first time - nearly impossible. And you want to do as few major reworks as possible.
An example build might be like:
- Design interior and hatch locations using sketchup make 2017
- Find out hatches aren't manufactured in the right sizes to fit design
- Redesign interior
- Nearly there, but can't fit water inlet
- Redesign interior
- Order hatches, doors, water and power inlets.
- Find hatch no longer made in that size
- Redesign interior for new hatch size
- Now I have all my hatches and an interior design
- Build interior
- Order shell (if you're super confident order this while you're building the interior)
- Shell arrives, glue it together, cut hatches/doors, glue in hatches doors
- Drop in the interior
- Quick and easy build with few headaches (hopefully!!)
Not sure if this is the right way to go, but it works for me.