I just bought one of these Six Pac campers for my short bed. It is a 1976 model and is just like yours.
The water fill is located in the far front left near the camper jack...and the 1-1/4" fill hose runs under the cushion to an elbow and then down to a holding tank that occupies the far front left cabinet on the floor. It was a generous 15 gallon tank. There was only a manual pump faucet used (and they frequently leak around the handle) so the wiring is likely from an aftermarket addition of an electric pump. The water line from the tank to the faucet is run under the lip of the bench (dinette) on the inside of the unit (not sure if this is original though.) It enters under the sink in the front right of the cabinet thru a hole at the bottom.
There is no grey water tank unless it was purchased as an option so the unit is designed to dump right over the side (I'm sure the Forest Service loves that!) I'm planning to fit a plastic container into one of the wheel well cubby holes to catch grey water. Even the 2002 models dump the grey water over the side!
The vent tube for the fresh water tank actually terminates inside the camper on the opposite side of the water fill just above the dinette cushions! The tube is just sitting there begging children to stuff jelly beans or used gum into it!
I considered replacing the icebox with a fridge but it really would be worth just getting a newer version of this camper that has the fridge since $1000 or so is required to do it. Getting block ice every few days is actually cheaper than the hassle of propane which only lasts about a week anyway. Also, running a propane line across the camper is a challenge that requires adding a frame to the bottom of the camper (needed for proper clearance anyway on newer trucks) so that the space can be used to run a 3/8" galvanized pipe across the bottom. Later models of this camper were manufactured this way.
This camper had no converter, battery compartment or fuse block that I know of. Three wires +12v,Gnd,Running Lights emerge from the left rear wall and plug into the pickup. Only the lights use electricity. There are no other electrical items in the camper. There may have been a factory battery shelf in the left rear corner behind the 7" wide skirting...or one could be added there...especially easy when adding the frame to the bottom of the camper that is needed for truck bed rail clearance on these units. Just make the rear board wide enough to reach the ends of the rear skirting and a battery can be added on each side! A narrower-than-normal battery is required such as the Walmart SLA battery so there is no interference with the truck bed opening. A solar system will keep them charged so the lack of a converter is not so bad.
