On US199, the tunnel is the least of your worries. It's plenty big. Prior to the tunnel, the highway follows a windy river. Some curves are tight enough that the state limits RV and bus lengths to 40ft - but with your trailer that shouldn't be an issue.
You might consider staying on US101 as you enter Oregon. The drive to Washington will be slower than on I5, but over all it will be more scenic. You see more of the coast in Oregon than in California. With Bellingham as your destination you could stay on US101 all the way to Port Townsend (and take a ferry to WA20), bypassing the big cities (Portland, Tacoma, Seattle) entirely. And then take WA20 over the Cascades on the way to Idaho.
US93 saves some miles, and potentially time, over taking I15, but the difference isn't great. Most of the time, mapping programs choose the fastest route, even if the time advantage is minor. Rarely does a recommendation mean 'this route is vastly superior to all others'. That said, plenty of posters like US93.
