I am having the same trouble on my 2013 Keystone Alpine 3720. First time completely full. The low point drains in the water center open, run for 30 seconds hard. then stop.
red & blue near the rear axle run for 10 seconds.
no other drain that I can find. opened faucets, ran pump. no change.
found a little water, quart, on the storage bay floor, pulled the panel behind the water center.. I don't see how the low point drains could drain the tank, as there is no container higher than the drains.
in the water center there are handles for the 2 grey/1 blank tanks.
the other day I noticed another unlabeled T handle just behind the water center, thru the chassis.
| edit.. found it.. behind the rear axles, driver side, another white valve.
| edit 2.. found the cause of the water on the storage floor..
the seal around the water center box seams had a small section (1 inch) where the seal was not complete. so water on the floor of the water center leaked thru the corner.
still don't know what the T handle does, tho I can guess its a grey tank drain.. but supposed to be only two grey tanks, and have two grey tank drain handles already..