I'm about to do the same, scraped the refeer tank idea due to lack of quality used and $$$$ for new. Bought a DeeZee aux. tank, aluminum diamond plate, 110 gal. for $600. Perfect dimensions for me as I wanted to keep my toolbox in the bed for the winter. This tank is 48"L x 24"H x 24"w, so it just fits between the wheel wells. I had to have a shop weld up another 3/4"NPT female drain at the rear of the tank, as it came stock with a side drain. Planning on running it with a ball valve and 3/4 line into the vent/overfill line.
Question... do you think a water/fuel seperator filter would be needed ? Just wondering if I might encounter a problem with the metal tank during winter. I plan on filling it once per week, I only use about 20-25 gallons a week normally. I'll add my Stanadyne PF right to the big tank. Any need for a biocide ???
Scott W.