In reply to JWTM:
caledonian campers in dumbarton (as mentioned above) and Rockin Vans in Kilmarnock seem to be two of the main players in the west, but they both seem to have big waiting lists at the moment. Aug for fitting two windows was what I was quoted recently. Mind you, Rockin Vans do amazing work, so it might be worth the wait.
Vanguard in fort william have a good rep apparently, but think they're closed for a move of location.