Cheap? RV park? If it has a lot of stars, it isn't cheap. If it doesn't have a lot of stars, it's a dump and nearly anything you leave outside will disappear, and it still won't be all that cheap. If you require hookups, you're going to pay a minimum of $20-35/day.
If you're self-contained, consider boondocking on BLM land, Army Corps of Engineers campgrounds, National Forest Service campgrounds, etc. But many of these don't have space for the bigger rigs (over 20 ft). If you're traveling around, many of the Tribal casinos will let you park in their lot, but check with management first.
In between, there are local parks and small campgrounds, often near water, charging $10/day w/no hookups available, but may have water and restrooms. The State Parks are in high demand and tend to require reservations at least for the main season.
I love Oregon (lived there for a while), but they also know they've got a good thing (all beaches are public) and they're prepared to charge you for it.
p.s. If you qualify as a senior, check out the Interagency Senior Pass for $10 per year. It's good in many places, but not State Parks. Having one at the right campgrounds can cut the fee in half. Check it out here:
http://www.fs.usda.gov/detail/umpqua/passes-permits/recreation/?cid=stelprdb5353054
