I actually used to have a 55-Gal Drum as a 'spare' fuel tank in my '77 Chevy 4X4 for extra capacity when we were taking deep 4X4 trips into the desert or Mexico years ago.
Since the truck already had the two (2) factory side tanks, the "Drum" gave me @90 Gals total ...
I didn't bother with any fancy switches/solenoids/valve(s) for this, just hooked a 'feed' line (with a positive Shut Off Valve in the hose line) to a 'T' in the Vent line at the filler tube, and then modified the right side tank sender/pick-up bung fitting by removing it and installing a new (solid line) 'pipe' into the removable sender base - think of it like a return line fitting, but it extended down about half of the tank depth.
Then I reassembled and routed the 'new' pipe fitting up to the 'Drum' as a "Vent" via fitting threaded and sealed into the upper area above the largest threaded bung.
I just used the larger Threaded Bung in the "Spare Drum" as my fill point - yes I had to unthread to fill, but this wasn't very often - and 'sealed' the drum back up once full again.
In operation - I filled the upper Drum, opened the lower outlet valve to feed the right tank (normally right as we left the paved/main roads and that tan was already down a bit) and as soon as that tank hit @half-full the new piping I had installed would get above the fuel level and then it allowed vent air to get to the spare drum in bed of truck and allow fuel flow into the lower tank ... viola! as the tank filled it covered the 'vent' and stopped flow for a while until fuel level dropped again and fuel would flow ... and so-on, and so-on, and so-on ... until my 55-gal drum was empty.
I would just monitor the dash fuel gauge - which would just stay @1/2 until the drum emptied - and as soon as it started to drop again I knew the 'spare drum' was empty ...
My "Range" was now really impressive
Seemed like we were always siphoning fuel from my rig to give others ... I once made it from L.A. to Albuquerque without stopping (okay, once to 'P'
) and really could have made it further, but didn't know where to get fuel next
This system worked flawlessly for years, never leaked, and required no maintenance. I had the drum strapped into bed on 'bunks' with metal banding that just unbolted when not needed to give me my whole bed back again.
I finally removed the plumbing for it about 10 years ago after many years of not going that far back offroad anymore ...
Soooo - if you can fully seal the 'spare' upper fuel cell/tank and use a vent tube that is positioned under normal upper fuel limits - you can have a tank that will 'vent' via the fuel level and not require any fancy pumps/valves or other items to transfer fuel and maintain level/flow.
The "Secret" is the 'seal' of the system