Were those "little drain pipes" for just your black and grey tanks? If so, no big deal. It's possible/probable your camper was winterized using a bypass for the water heater.
To sanitize the fresh water tank:
A quarter cup (1/4c) of bleach per each 15 gallons of water.
For better results, mix the bleach into water before putting it into the fresh tank instead of adding water and and dumping in bleach. (Others will disagree, of course.)
Let it sit awhile. Drain. Rinse (use baking soda in the rinse if you want). Repeat as many times as needed.
If you have a filtration system, remove the filter. Same for the fridge.