My plan is to use a re-purposed water barrel. I'll use a small 12V pump to transfer the grey water from my on-board tank into the barrel, sitting in the bed of my truck. Then just reverse the process at the dump station (or at home). The whole thing should give me about 50 gallons of extra capacity and cost about $50 including pump and barrel.
Downside: no black water. But we can usually get enough time out of our black tank to make it work. Because DW likes her long showers, it's the gray water that fills up LONG before the black anyway. And we can extend the usage of the black tank by using CG facilities when available.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!