Since I have a side radiator, I had room aft of the motor and below the level of the rear access door to the engine compartment. I added a 15 gal ag sprayer tank and a small DC pump. The tank fits there nicely and has a top that is easily accessed from the driver side truck stop DEF dispensing pumps. Tank and pump was $79 at Tractor supply and is completely non-reactive to the DEF. Farmers spray urea solutions all the time with these tanks and pumps.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad