That seems like a nutty "rule" to me. The manufacturer expects you to fill up with 120 gallons of water for a quick weekend trip and then consume every single drop? I doubt your two waste tanks add up to 120 gallons, so that's a lot of outdoor peeing if you're going to abide by their "rule".
Is there a valve (or can you install one) between the two tanks? It would be a lot easier to manage a full/empty 60 gallon tank.
When we were new with our first TH twelve years ago I would try to drive to the destination empty and then fill up with water once we got there. It was such a hassle and it added way too much time to the drive. I would also dump unused water before driving home, but what a complete waste.
After that I started using a gauge on the fill hose and fill up the amount of water I thought we needed based on the number of people, number of days, and how dirty I expected to get (showers?). That worked pretty well.