Regarding your grey tank size - how big is your black tank?
As you camp, you can off-load your grey tank into the black tank each time the grey tank gets near the top. We carry along a 3-gallon plastic bucket to fill from the grey tank outlet and then empty into the black tank periodically whenever the grey tank gets near the top. I carry the bucket full of grey water into the bathroom and dump it down the toilet. This effectively makes the grey tank a lot "bigger". You can even conserve fresh water by dumping grey water into the toilet whenever you want to flush the toilet so that you're not using freshwater to flush the toilet. We wash up ourselves a lot with desposable pre-moisted towelettes instead of taking showers or using precious fresh water in a sink.
Of course to boondock you just have to first to learn to drycamp in your RV ... it's not rocket science to drycamp and is a lot of fun. In my opinion that's what an RV with tanks, a battery, and propane is "for".