I did exactly what Old-Biscuit stated on my Thetford.
There is a stub on the back of the toilet sticking down from the fill valve. I cut the tip off the stub (like opening a tube of caulking) and used a hose clamp and a kitchen sink sprayer that I bought at Home Depot. The sprayer is only pressurized when I am holding the flush valve open. I just lay the sprayer over the edge of the tub when not in use in case it leaks.
The boys at the time were 6-13 have never messed with it, but then again I don't think I've ever shown them how it works. So they've probably squeezed the trigger and when nothing came out they just lost interest. Then again it's kind of an awkward angle to try bend over and hold the flush calve and then point the sprayer at somebody as you're body is mostly in the way. That's probably the most realistic reason why they don't spray at each other.