Since our Bus has a side radiator, and there is considerable room between the rear cap and the engine, I installed a 15 gal poly ag spray tank in the space. The tank has a small 12v pump that I use to run a rubber hose to the Bus spray tank. Since we travel to Alaska every 2 years and there is little or NO bulk DEF once you get north of the lower 48, the system allows a fill in Montana on the way up and another in the same place on the way down. The aux tank fill is in the center rear of the rig and DEF pump hose reaches it easily.
I know all the warnings about corrosion and age of the DEF, but neither apply. THe pump and tank fittings are all made for agriculture spraying of nitrogen fertilizer which is very close to DEF and the DEF isn't ever in the tank more than 12 months. We've been using the setup for almost 3 years and it works great.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad