The more i think about it, the more i like the crossover siphon option (upper picture in last post)
Best i can tell from web forums it sounds like semi trucks and boats often use a similar strategy.
https://www.thetruckersreport.com/truckingindustryforum/threads/the-fuel-crossover-line.109607/The only variations i need to still think about:
1) connect fuel pump suction line to crossover line
2) ... or connect return to crossover instead
3) don't connect anything to the crossover
I'm leaning towards hooking the return line to crossover just in case it gets bubbles when tanks run low and start sloshing
The main tank does not have a positive vent,... so I'll probably have to drill a little hole in it's fill cap, and then put a vent line on the aux tank to make sure there's no pressure difference.
