Possible locations for gas leak

[BuggTruck]

So always had a leak, but now am going to plan the repair and add it to my list of things to do.
So currently i use only the front tank and do not fill it. I only drive short distances right now and avoid any leaks this way. I believe the leak to be my filler necks on both tanks.
If either tank is filled it will leak. If im on rear tank, after i shut everything off, i get a leak (as gas drains back into tank?). I do get siphoning between tanks when both tanks are being used and lines are primed.
My switcher is fine, gas gauges are fine. Everything is functional.
So other than the filler necks, other possibilities Could be, seal around fuel pump or gas lines, or related connections above tank i cant see.
Anything ive missed?

[ScubaSteve]

Make sure your tanks are vented. I once had a problem where both of my tanks could not vent and would build pressure when the truck was stopped. This would push gas out around the O-ring on the sending unit if the tanks were full. If you get a gush of air when you take off the gas cap this could be your problem.

[Whisler]

I had a leak only when tank was fairly full. When the tank was pulled it was discovered that there was a small rust hole in the top of the tank.