i would asume as they where called 4 tonne bedfords the load would be 4t, as for fitting it for a camper we used to have one as a fitter truck before the SV fleet came along. ours had alot of isulation, four beds ( 2 bunkbeds) kero heater full work bench, and a double cabinet set up, ontop of all that we had five or six full toolboxes, 2 tiffy boxes, various other stte, kit for a minimum of four people for a month, expense alot of it, P.O.L various, cooker,tv etc so weight should not be a problem.
at the end of the bed we had an aluminium sheet covering the back with a sliding door to allow entry, the tail gate was always hung off the chains to allow for extra space and a set of steps allowed us to climb up the back.