Can anyone explain the 4005 tank situation. I pretty sure are two gray tanks, but not sure where the dump position is for the rear one. Also I assume there is one black tank and the front toilet waste flows the length of the unit to a rear tank that also gets the garage 'john'. There are multiple valves and non seem to be labeled.
What does the small sticker reading 'low point drains' is next to gray tank #1 door mean?
Anyone know of a diagram explaining the whole setup?
Forget the diagram. Even if there was one the odds are good it would not be model specific and probably would have changed anyway.
I don't know about your exact model, but many of the toy haulers with the rear bath have:
1 gray tank for front bath shower and sink
1 black tank for front bath toilet
1 gray tank for kitchen galley sink
1 black tank for rear bath toilet and sink
You probably have two waste handles somewhere near the front docking station and two more somewhere behind the rear tire (left side or street side)
Even if labeled, odds are good they are labeled wrong. Most on the forum here just fill the tanks one by one to see where the water comes from when pulling handles.
"Low point drains" - means exactly what it says, these are valves that "should" be the lowest point on your water system to allow draining the water lines when you winterize.
But, just because they are low point drains, don't assume they are actually going to drain all the water until you prove otherwise. Many times the PEX water lines are routed with dips that do not drain.