Some RB44's have three seats in the cab, the drivers seat is nicely sprung and can be reclined and lowered to suit leg length and height. The passenger seat is a fixed backed affair which is completely unsuitable for man or dog. If you fitted another 'proper' seat in its place it wouldn't be a three seater any more.
Space wise in the back you could sleep four under canvas with two adults across the load bed and children inbetween the seats but you would run out of space in the back if you wanted to add a toilet, a shower or proper beds.
Cab is a none starter not enough usable seats
Space in back not enough for what you're after
But as you said its a winner on the driving licence side but it falls down for the reasons above.
Someone near me has done something very similar to that if not the same
DAF details
Power Steering,
Hi/Lo diff lock,
3 seats,
air brakes,
rear Nato style hitch
Length: 6.65m Width: 2.49m
Height: 3.43m Weight: 6500Kgs
Withams must be taking the vehicles weight directly from the plate in the cab??