Hey Marty,
It's been a few years since I've been down there, but as far as the mud goes..... you'll not likely sink up to your eyeballs, but it'd be highly reccomended to bring along a pair of old sneakers or cheap rubber boots to use in the boat to keep the sticky stuff from following you to your tent. This of course is dependant on water levels and weather. But, better to be safe than sorry.
I myself have never camped at Mossyrock, but what I remember seeing is "combat" camping at it's finest. They're just small spaces in a big open field. But it is Memorial Day weekend afterall, so's finding solitude would be an exercise in futility anywhere you go.
I've never had much luck fishing the main lake. I've always been under the impression that you'd need a downrigger for that.
When I'm there I usually stick to the numerous coves or sunken islands on either shoreline and have fair to good luck trolling for 'bows,'cutts, and, 'kokes.(mostly used 1/8th oz. black/silver Roostertails) I usually fished this lake in the winter so's hopefully someone with greater experience will step forward and cue you(and me!!!) in on the finer points of fishing this lake.
I would however, like to "strongly" advise you to aquire a good map of the lake and surrounding area, as well as, having emergency/survival gear in your boat. This is a big area, and aside from the campgrounds on either end, it is all undeveloped foresty land and wilderness.
If, for some reason you find Riffe too daunting a code to crack, you could always give nearby Swafford Pond a try. Although it's known as trophy panfish waters, it is stocked with 'bows, dutchies(brown trout), and channel cats. It is also "small boat" friendly(-no gas motors-).
I'm not sure how old your boys are, but there is also some really good "rock hounding" at Riffe. The shorelines are littered with quartz crystals and various other rocks and minerals(there's even a chance of finding a "thunder egg") and if you look hard enough there are also fossils. What little I've found is of an extinct species of Ginko tree(about 100,000 years old). Also, don't forget your camera!!! This is some exceptionally "photogenic" country.
I guess the only other thing I can tell ya is that this is "Sasquatch"(Bigfoot) country, so you might consider adjusting your campfire stories accordingly.
Hopefully, a local/area pro will chime in with better details. Good luck and tight lines!!!