Luckily the three fitting senders can be had in carbed or TBI versions, so either way you are covered there, right? No way around dropping a tank I can see.
It's unfortunate it would be so much additional work, but adding a fitting on top of the primary tank, with a drop tube brazed to it inside the tank running to the bottom, would give you seamless/constant fuel transfer from the other saddle tank as it would automatically siphon/level off as the primary tank draws down. No fuel gauge for the other saddle tank needed. You'd just need to run a sealed cap on the primary tank.
For the tank in the bed that can't be applied, but two senders/pumps is easily controlled.
However. If you tied the two saddle tanks together so that they self level, if you ran the bed tank to the primary tank, as long as the saddle tanks were at, say, 1/4 tank/24 gallons (which the primary tank gauge would tell you, no need to monitor both tank levels as they would always be the same) then you could dump all 20 gallons without concern for overfilling the saddle tanks, nor the pump in the bed tank, since you said it self empties once it starts flowing with the pump so the pump is run only to get a siphon started.
A bit of "risk" not being able to see the bed tanks level, but if you only use it full to empty, and not partially, then it wouldn't matter. It's 20 gallons, you already know you can estimate that will get you X miles.
If you want to play around with the concept yourself, it's easy to do with some water, rubber hose, and plastic bottles with caps serving as fuel tank stand ins.
