Mt Rainier is the only place I’ve ever heard of which used a tape measure and told a couple setup that their 5er trailer was too long and they had to pack up and leave.
An NPS ranger I know works the campgrounds in the summer and told me the Superintendent of the park a few years ago got very strict on enforcement of the length.
The main problem at Rainier is the very tight campground loops with almost no clearance in the tight turns. 5ers that cut farther to the inside of a 70 or 85 degree turn with trees on either side and a 10 ft wide road. When I visited Rainier last June and talked to Maria, they were up to seven trailers since the campground opened which had gotten stuck between trees and required wreckers to clear.
Of course, such a stuck trailer means no one can come in or exit that loop for several hours.
I won’t tell you that your rig will fit or not, or that the park ranger might deny you a spot because their measurements say it is too long.
I will tell you that there is/was no way that I would travel from Medina Lake to Rainier planning to stay in a park campground if there was any question. We found several nice campgrounds close to the park.
We stayed at a nice municipal park on the lake in Mineral. Dry camping just like the NPS campground.