I've been looking around alot and I think a Yeti 105 will fit 2x2.5gal kegs and a dual body regulator and a paintball CO2 (for serving only). I just don't think there is any cooler out there that will fit a 5lb bottle.
It's interior dimensions are (L)23.75" x (W)13.625" x (H)15.313"
Of course it's a ridiculously expensive cooler, but its the only cooler I've found that will fit what I'm looking to do. Not sure if I'm going to go down this road, but it seems to fit the bill.