Marko, I was thinking of commercial too, but I'm going to redo the cab as well. since that floor is not nice and flat we need to think about the carpet we do buy. I have done a lot of my own carpet, mainly stairs, closets, doggie steps and even my 1978 International Scout 2 (which I wish I kept). Anyway, much commercial carpet has pretty stiff backing. That can be worked around my moistening the backing where you need to make a bend. We have the 'Latitude Haze interior, and since we are reusing almost all of the current upholstery, I'm thinking of a rug in the 'dirt color' (medium brown) family with some kind of pile friendly to bare feet. I'm sure we will find a remnant somewhere. However the stiff backing then raises it's ugly :M head because the older the carpet the stiffer it gets. I already looked at the roll goods in Lowes, but I'm going to keep looking.
I will be laying the carpet in all areas except under the new box. No reason to have the water piping or tank sitting on it. If there is a leak which is what happened to us when we got it, the rug wicked the moisture out all over the living area and ruined the panels on the original dinette seat.