At Millersylvania you are about 5 miles from either 99 or I-5 and last time we were there we didn't have any trouble with our Verizon. I didn't have my Bionic then so I don't know about the hotspot. We have also stayed at Olympia rv park, a former KOA and it's a just ok place. It is a bit handier to town though.
When you're there,cut over to Tumwater Blvd and checkout Eagan's burgers.Been eating those since I was in high school in the late 50's.
Potlach is so far away,that you'd be better off staying in Seattle.There is a park at the bottom of the Nisqually hill,but I wouldn't stay there and then a bit farther east is Lost Lake.It is surprising how few rv parks there are there.