We've stopped into RV dealers to ask where a dump/water site is and been invited to use theirs at no cost. They usually say to remember them when we're ready to trade.
We also look online...g00gle shows several websites under rv dump sites.
Many city dumps/fairgrounds usually go for $3-5, some free. Pilot/flyingJ is $5 with their club card. RV parks usually $10 for dump and water if you're not staying.
When in the boonies, we shower and do dishes outside if possible since our gray tank is smaller than the black, which can go up to 12 days for us....especially if we can go out to pee.
Once in a small town, I asked at a little store where there was a dump. The guy said he dumps his in the sewer cleanout pipe on the side of the building...."help your self, just keep it clean", says he.