I can't believe I didn't see this post earlier. I currently use 5 gallon cans, which works well because I burn about 15 gallons a day. It's a pain in the butt since I have to stop and fill the cans every day. I just filled a 60 gallon poly drum that I will carry on the trailer and will use a hand pump to fuel with.
I want to have a custom steel tank made to mount underneath the deck of my trailer (bumper pull). There is plenty of room underneath the deck and I have good ground clearance. I was thinking of mounting it just ahead of the axles. I was thinking of tapering the bottom to a place to plumb in a 12 volt fuel pump (also mounted underneath the trailer). Since it will be sucking off the bottom of the tank I will have a in-line filter and water separator after the pump. I was going to mount a deep cycle battery (will charge through the trailer harness) in the tongue area of the trailer. I would also store the hose in the tongue area. Every thing should be out of the way and secure. I was thinking about a 100 or so gallon sized tank. This would be about 7' wide (inside the frame rails), 1' deep, and 2' long, but may have to be shallower for ground clearance (volume easily made up in length). Diesel is about 7# / gallon, the tank full of fuel, will add about 1000# to the trailer.
If you have a gooseneck trailer, a similar setup would work well in the gooseneck area. It could even be gravity feed and you wouldn't need the pump.