l am trying to achieve dual fuel tanks for extended driving range.

You could put a fuel cell in the bed..if you went under the pickup it might be a tight squeeze to get a cell of any size under there.

Not sure if you can add another tank, but l think you can get an optional 44 gallon tank and replace your 26 gallon one, Good Luck

There r several choices. There is some company that makes a mirror of the driver side tank for the passenger side of the vehicle. Dont know if they make it for ur 07 dmax. l am assuming that yours is a 6 1/2 ft. bed. An 8 foot bed would be +-36 gal./ Some company (mabe the same one) makes a 36 gal. tank for ur truck im sure. l have a dmax as well. l opted for the L shaped tank in the bed. The cross box fits well with it & it holds about 100 gal. l have a range of around 2350 miles between fill ups. It also is easy to install. Just run a hose from the l tank to the vent hole in ur existing filler cap (under the bed of course). It will keep the tank full untill u run the big tank out.