My used Dream Coach already had the tank and pump in it. The place I got it from had originally sold it new, and they sell the accessories as well. Check out Blue Ridge Trailers. Somewhere on their site they offer water tanks, pumps, awnings, etc.
I'm looking for input/suggestions on what kind to get and how many GPM I should be looking at.
For a proper answer, you must provide your water needs and the size of the supply tank. Are you wishing to wash a horse? Or just rinse bits and bridle. Hand washing or a shower in a dry camp? Drinking water in a bucket for a horse or two? or three? or four?