So dry fitted the floors and then placed the profiles on top just to check everything fitted. Lined up the back edge of the porfile with the back of the chassis and I immediately realised my mistake.
Because I have chosen not to bring the curve back in under the hatch at the bottom it means the profile is about 4 inches longer at the bottom than the drawing that I had cut the chassis to, so the profiles stuck out 4 inches in front of the floor

Good job I hadnt glued or screwed anything down at this point. So adjusted the front board to stick out 4 inches over the front of the chassis and realised the board cut around the wheel arches was now cut in the wrong place.

Now came the choice, leave the board around the wheel arches and put a 4 inch strip in or cut the 3rd board to replace the one around the wheel arches then either order another board to fit at the back or cut the offcuts I had to make up a board to fit across the back under the galley. Being the frugal sort that I am and having just acquired a biscuit jointer I decided to use the offcuts. Truth be told its turned out OK.
With the profiles clamped in place I put a cargo strap around the top to hold them the right distance apart and with my other new acquisition (a mitre saw stand) I cut all the beams for the roof and dry fitted them. I placed a board in the position the galley wall will be and tried the sleeping area for size with SWMBO next to me. Plenty roomy we have enough width for a UK kingsize mattress (5ft wide 6'6" long) and 3 inches extra length which will come in handy as I am 6'4" and my feet often hang off the end of our normal bed. Will probably have to put 3" of foam along the bottom to stop things moving about, the other option being getting some foam cut to the exact size and get her indoors to make a fitted bottom sheet.
So with that sorted everything was stripped down again and the board for the floor got another coat of International ranch paint and left to dry before biscuiting, gluing and screwing down tonight.