I bought the Harbor Freight transfer pump. You can connect a female garden hose fitting to both the in and out, or a male 3/8" fitting. Since I removed the fridge a couple years ago I installed the pump just inside the external door that provided access to the guts of the fridge. There is enough room in there for me to connect a hose for input and output. I still had the fridge wires sitting in there so I hooked it up with a little switch I had. I plan on putting the shower tent right next to the camper/pump from which one can reach through an opening in the tent and switch the pump on and off. The input hose will simply run about 5' into the water carrier that lives on the back bumper. The output I haven't quite figured yet, a piece of garden hose would be fine for filling the camper H2O, not sure what I'll use for the shower head or whether I'll need to install a valve to regulate, I assume so. I'll play with it tomorrow and report back.
Update on actual use: I haven't been using my 4Wheel as much, got a new woman and that required a fancy truck camper, a 9.5 Lance with AC and a wet bath, but I just finished a trip with the FWC and used the transfer pump as a shower and as a great way to refill the Grandby watertank. I usually carry an extra water jug on the rear bumper, holds 6 or 7 gallons. To get that water into the Grandby I just drop one hose from the pump into that container and the other hose from the pump goes into to Grandby tank. Hit the switch and it pumps, no more lifting or getting water all over me. i don't even need to move the extra tank off the bumper until I want to refill it. Mine is hardwired and the switch is accessible via the old fridge access door but it could easily be made as a portable unit as well using a cig lighter or other electrical connection. If you want a hot shower I just heat a pot on the stove and add it to a bucket and pump straight from there.
