Prompted by Rae's thread where he's re-use some wood, as I've done a bit of the same I thought I'd list the not full price stuff we've used:
Kitchen units from eBay, that came with the integrated fridge. Most of the carcasses have been cut about to suit.
Solar panels off Phil's van. ( just don't tell him lol )
Nigel Brown used old furniture and all sorts to make the table. Also bought the toilet access door from him, I think this is at least the 3rd vehicle it's been on
Kitchen worktop - cheap from one of the trade places I use as it was damaged on both ends. I just had to cut it a couple of times more than necessary.
Curtains, eBay and a mate trimmed/added/bodged to make the two for the kitchen.
Carpet - offcut from eBay and some (new offcut) from the skip at a supplier.
Vinyl floor - another supplier essentially gave me this when I told him why I was building the truck.
Shelf above sink - part of an old table that came with the truck.
Mattress - never used, but second hand from eBay.
Lights that would have been binned re-used under kitchen wall cabinets.
One of the big doors came from a job where I replaced some doors for someone.
Tv in Tommy's room - out of his home bedroom as it was too small for him to see and he got a new one for Christmas. Our tv is out of the old van.
I'm sure there is more lol.