Does the 356QBQ not have low point drains?
It has a fresh tank drain and the vent tube has a valve on it but unless the low points are hidden well, I don't see any.
Hope you found them. Mine are directly above the black/grey tank drain pipe, behind skirting.
What I can not find, is if there is a water filter installed somewhere...
Actually your vent tube doesn't have a valve on it, that is the fresh tank drain. The vent shouldn't even be run down, FR should have them run up to the vent connection next to the fill spout - a mod I've done.