Hot floor between the cabinets in a '73 Westy? Sounds to me like the heat
 is on. 

 When the red lever that opens the flapper valves is pulled down, a portion
 of the heated air is ducted to a small "grille" located between and just
 behind the front seats, exactly where the original poster had noticed the
 hot floor. This duct is switched on and off by the defroster control - When
 the red lever for defrost is pulled down, this "between-the-seats" duct is
 shut off, diverting max heat flow to the windshield. 

 The valve is operated by a relay cable from the defroster flapper in that
 tube at the very front of the bus. Perhaps the age and/or condition of this
 cable has caused the valve to stick open but, more likely, there is no 
 problem at all. Unless the floor is getting hot when the heat is off, which
 would indicate the the flapper valves on the heat exchangers may not be 
 closing completely. I've been tempted more than once to dis-connect the
 cable and divert all that precious heat to the front, but I have a feeling
 that my rear seat passengers might object, that grille being their only
 source of hot air, other than the driver's conversation.

 Westfalia campers in '73 were equipped with an icebox, so the camping 
 equipment is unlikely to be the source of heat unless an owner has installed
 a refrigerator subsequently. '74 was the first year that Wesfaliawerke 
 put refrigerators in the campers for the US market and those were electric
 only units.

 Best wishes for an enjoyable Holiday,

 Dave