What size is that vent hose that goes up to the filler? Looking into tapping into that for running the outlet of the in-bed tank into.

Any help would be great.

5/8

That's how Transfer Flow's system is set up. They provide a "double-T" to be plugged into the OEM vent hose. They also vent their tank to the OEM. Maybe you could get one of their fittings from them if you don't mention that you are using it on a competitor's tank. ;)

Thanks guys! I like the vent idea too...
Now off to really engineer it...