I know on my Flagstaff water tank, the drain is in one corner and you have to crawl underneath to see it the first time. Once you know where it is, you can open/close just by reaching underneath.
As for the water drain on the side, that might be for grey water from the faucet...maybe.
Trailer: Lifted 228BH, heavy duty springs and Yokohama tires
TV: 2016 GMC Sierra Z71 4x4 CC, SLT
Spare TV: Two Alaskan Malamutes
Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!