I have never been in this situation but I really like this idea of using a 12VDC MERCATOR PUMP and pump your waste directly into a portable tank in the back of your truck bed. Always have a portable tank larger than the tank you are pumping. When it gets done just unhook your hose from the pump and tie off high and drive to dump site. Then let gravity feed empty your portable tank...
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FLOJET 18555-000 Waste Water Pump Kit
Watched one guy do this at a campground and all was easy completed with no mess whatsoever.
Roy Ken