Depending on state laws, you can use a hose to run gray water away from the camper and just dump it. It's just water with some food and maybe a little soap in it, totally harmless...unless you're in Oregon where it's more toxic than atomic waste.
You can also wash dishes in a plastic pan and dump it somewhere to save room in the tank.
If needed, you can pick up several 5 gal jugs for extra water and carry them in the pickup. My water tank only holds 35 gal and they're definitely needed.
They probably say to leave a 70 gal tank half empty because of it's location. If it's too far back, the weight can make the trailer sway. I've been thinking about hanging a 2d fresh water tank underneath somewhere around the axles. This wouldn't be used in cold weather, obviously. Some creative plumbing can make it easy to fill and a valve by the water pump can make an easy switchover to pull the water out of it.
