When we had the popup it didn't have a gray tank, just the drain outlet on the side and a 20 gal. fresh tank. I set up two Wedco 5 gal. tanks so that they were in series via a hose barb that I placed in the vent hole I drilled out. This allowed one to fill and then overflow into the second. The cap was left open to vent on the second. It was quick to change over the inlet hose from the full one to the partial so that you could dump the full one.
I also had a folding luggage cart that let the kids take the gray water to dump it and they could take an empty fresh water tote to fill and bring back on the cart. 5 gallons out, 5 gallons in, unless it was getting close to when we would leave.
Early 2016 Rockwood Roo 19