Okay, so after thinking about it for a long time, I decided to just figure all this out once and for all.
For reference, my trailer is a 2014 Jay Flight 28 BHBE. My trailer has 86 gallons fresh water capacity (2-40 gal tanks, and the HWH). Since my trailer is completely open on the bottom, everything is relatively easy to see and trace.
The tanks are plumbed as follows: Each tank has a drain in the absolute bottom of the tank that flows to two 1/2 turn plastic valves labeled on the outside skirting of the trailer as a "Low Point Drain". Teed into the drain line from the rearward tank is the water pump pickup. This is all on the curb-side of the trailer.
On the street-side of the tanks, there is a 1.5" (?) equalization hose that is tapped into the side of each tank. This hose simply runs from one tank to the other and is there so that the tanks will fill "equally".
At the top of the tanks on the street side, there are two 1/2" (?) hoses to equalize air between the tanks; they also just run from one tank to another. At the top of the forward tank only are two air vent hoses that route behind the steps and allow air to enter and escape as the tank is filled or emptied.
Sorry about the low-light conditions of the pictures, I did this at about 9:00 at night after loading out my trailer for a trip.
This is from the street-side, looking at the rearward tank. You can see where the tank drain is tapped on the far left, and the line you see that looks like it's going into the propane block (teed between the tank drain and the tee for the drain valve) is the water pump pickup line, it goes straight up to the pump which is under the sink. You can see the rearward drain valve in the upper right foreground.
Here you can see the forward tank drain, and the forward tank drain valve.
This is the water equalization hose on the street side of the tanks.
These are the air equalization hoses (also on the street side) between the two tanks.
This is back on the curb-side from under the stairs. These are the two air vent hoses tapped into the top of the forward tank.
The fresh water gravity fill is up on the curb-side of the trailer above all this and goes into the top of the tank like normal, with it's standard vent line.
So, there you have it. Mystery solved, sort of. This is how it's plumbed on my trailer, as they say, YMMV.
The one big question I STILL have is this: Because of the location of the pump pickup line (teed into the primary tank's drain line), the pump should always be able to pick up water if there is water in the tanks that can be drained.
However, this is not the case. The pump will stop pulling water LONG before the tanks stop draining. And not just one drain, but both drains, and more importantly, the rearward drain where the pump pickup line is plumbed. How can this be?
The only thing I can figure is that there is not enough pressure? The pump is pulling more water than can be drained?
I have some mod ideas brewing here to make all this better ...