~83 miles to T-Mobile park from there, but it's all freeway, and it's on the drier side of Snoqualmie Pass if we get rain that weekend.
The OP will be visiting Seattle for baseball games, so it doesn't really matter what the weather will be somewhere that has weather that isn't the same as Seattle's. And even on the freeway, driving 83 miles is going to take a good bit of time.
What about Lake Pleasant RV Park in Bothell? That would seem to meet your standards, and it's a lot closer than Cle Elum. In my experience it's the nicest RV park in the area.
I realize that the 4th of July weekend is also going on but we will be in on the 5th.
The 4th is on a Thursday, and it's possible the following weekend will be considered by many to be part of the holiday.
Are you going to all three games? If so, and if it were me, I would prioritize location, and I would seriously consider Trailer Inns, but my standards aren't that high if I just need a place for the RV to be while I'm running around. I remember driving by Issaquah Village, a little farther out than Trailer Inns, and it looked very acceptable.
Maybe I've been lucky, but I've been fulltiming for 20 years now and don't remember every staying somewhere I didn't feel safe. There have been skanky places, and ones that had visits from the po po during the night, but I've never thought somebody was going to steal my car or rob me. (But speaking of, Issaquah Village is right near Lake Sammamish, where Ted Bundy famously plied his trade; I would suggest not accepting a help request from a good looking guy with his arm in a sling and a Volkswagen with no passenger seat.)
I stayed at the Tulalip Casino about 10 years ago. I don't know anything about current conditions, but back then it was free; you just checked in with security. There was a 7-day limit, and I wouldn't be surprised if it's three days now. It's not like some casinos, where people park around the edge of a big lot--they have spaces marked out and they are very close together. Like parking lot close. But the price was right, and we were doing things in that vicinity (not downtown).
I suspect there wouldn't be a problem with availability at the Puyallup Fairgrounds, especially because they don't have site-specific reservations (yay!!!). That could very well be your only choice if people treat that weekend as a holiday, and it's a fairgrounds and not scenic, but acceptable as a base of operations, especially in a pinch.
But even better, it's close to Tacoma, where there's a place called Freighthouse Square, and I'm telling you, the guy running Paya Thai Fish & Chips and the two guys running Little India Express are the friendliest people I've ever met. I was right down the street for a couple of months, and I looked forward to going there just because of them and their salmon special at Paya Thai and the royal biryani and coconut curry chicken at the Indian place, which also had a divine salad dressing that they said they make themselves. I love that place.
There are some RV parks on the highway toward Everett, but they're in a pretty sketchy part of town, and a 40-foot fifth wheel could be a challenge--they weren't built for big rigs. I went through a couple of them and nixed them.
