Since you are headed up the Cascade River Rd, I assume you aren't that big of a rig. So the 'RV' friendliness of I5 through the cities isn't much of an issue.
And with Marblemount as your destination, I don't think US97 adds any speed advantage.
Google Maps says US97 to Wintrop WA20adds 80 miles and 2.5 hrs compared to I5 to WA20 (west bound). I think that's realistic. Rush hour in Seattle might cut that advantage by half a hour.
Consider I5 on the way up when time is important, and the east side on the way back.
http://www.nps.gov/noca/planyourvisit/road-conditions.htmThis says the construction on the CCRR is the last 3 miles, basically access to the Cascade Pass trailhead. By that point the road is your basic forest service gravel road. Do you plan on camping at one of the main NP campgrounds like Newhallem, and drive up CCRR in the truck? The main other hiking destination on the CCRR is Hidden Lake, which has a narrower and steeper access road.
https://www.flickr.com/photos/northcascadesnationalpark/sets/72157635049191212/are photos of the Boston Creek washout that is due to get a more permanent repair this fall.
