Why not just put in a coin operated shower as is used in some state parks?
I got 7 1/2 minutes of shower from a quarter 12 years ago at San Clemente State park. I got real good at showering shaving and rinsing off before the water quit, all on one quarter.
I remember when it cost a quarter to watch TV for a few hours in some hotels back in 1956, and a vibrating bed also cost a quarter.
those were the days eh?