Tool box combos are very nice. I have a 50 gallon tank with a separate tool chest box. Lots of room, the storage box sits on the bed floor. I fill that sucker up with tools, oils, grease, ratchet straps, draw bars, hitch cover, jumper cables ect. They make L tank combos with 100 gallon capacity, but the storage inside is much less. ATC brand was my choice. I added the transfer pump and filter for my Kubota tractor. ATC had a nice option list, tank and box was made to order. Gravity feed is the best method IMO. I've done the selector valve switch with the in tank pump/sending unit, it wasn't worth the hassle.
Having the extra 50 gallons pushes my towing range out to 800 miles or more. Most of my trips are under 500 miles, so I enjoy not having to stop for fuel with the 5th wheel attached. I can run to the nearest beaches and come back on one fill up. Its a nice luxury. :)