Forget that. Go to:
http://freecampsites.net
http://rvdumpsites.net
Privately owned RV parks, not the snobby resorts for people with 6 figure salaries, will often let you dump for about $10 if you can't find anywhere else.
I used one in Minnesota last year when I had the crappy ol trailer. They let me take a shower in their bathhouse in addition to dumping and getting water, all for something around $10 to $15. Have cash to avoid them losing to card fees since they are doing you a favor. They might just charge more to run a card though if you don't have cash.
That saved me from immediately undoing the action of dumping. And it was far cheaper than their $30 to 40 per night rate.