Depending on where you're coming from and perhaps travelling to; Alder Lake is a good State (quasi-state really) facility. It is a certainly off major travel routes, but we chose to take it in as a stop-over travelling East to see Yakima, WA.
On this note; a word of warning... if you are travelling East or West between Olympia and Yakima, you will likely be passing through Mt Rainier National Park and Snoqualmie National Forest -- neither are RV friendly passages -- a lot of narrow windy roadways with dramatic shifts in elevation. We pulled a 3,600 kg (8,000 lbs) RV, and we laboured through the entire passage. Having said that, some of the views were spectacular.
Alder Lake is a dam reservoir in reality. As a result, low sloping grades around the perimeter is not something this lake offers. Access points are 'developed' and spread around the lake at various points. The lake is popular with power boaters (we didn't have a boat with us, so I don't know what the launch fees were) with a decent launch ramp and moderate truck/trailer parking (for fee).
The campground is located across a small road from the primary day-use picnic and swimming area. From any campground in this location, it would be a 2-min walk. The campground layout is standard loops, with cut-away pads (asphalt paved), gravel area with steel ring fire-pits. At the time we were there, wood burning was permitted. Wood was available for purchase as well. The majority of the pads were not long and level -- bring levelling pads for your RV. While these campground sites are cut-away, there was still a lot of light available. There were a few walking trails to explore, and if you're adventurous, I believe there are full lake perimeter trails.
The swimming area was a real positive for the hot days. A very large grassy area for picnics and sports is affronted by a man-made beach to the swimming area. Swimming was cool and refreshing. The kids had a great time trying to walk around the log swimming guard. The area also has a depth market, which the kids would climb, then do flips off of. We would spend good portions of the day at the beach. There are a few attractions to take in in the area - Northwest Trek Wildlife Park being one.
The closest food/ice is available in Eatonville. If you are looking for broader options there is a Wal-Mart in Graham (45 mins from the campground).
The campground is small, so ensure you have reservations. Driving all that way risking a no-vacancy would be frustrating. Once you're there, you and your kids will enjoy it.