We've been using a simple pressurized, "weed sprayer" shower in our Airstream and it works great!
We purchased the container new then simply added a low flow head with a shut-off valve. I opted for a metal container so I could have in powdered coated black so in the summer we could just place it in the sun to heat the water. Right now we boil 1 part water then dilute with 2 parts cold water for a 2 gallon shower that lasts 7min of constant flow.
With this metal container it could even be placed directly on a fire/burner and heated if your didn't want to mess with transferring water from a kettle to the container.
You can see our setup towards the end (around minute 8 of this video if your interested:
Cheers!
-NICK