Just last fall we did a loop from Seattle down I-5 to Anlauf then took 38 out to 101 on the coast at Reedsport / Winchester Bay, then 101 south to Bandon, back to I-5 on 42S and 42 through Coquille, and then Seattle. We plan to do lots more of that part of the Oregon Coast, so were paying attention to the route trailer-wise. Both 38 and 42S/42 are perfectly fine 2-lane country roads, but 38 was more open, i.e. fewer of the tighter curves, lots of passing and pull-over-to let-people-pass spots, relatively flat, and better scenery; we could do the 50/55 mph limit all the way to 101. On 42S/42 the curves were lots tighter in places, it seemed more closed in, and there were some pretty good hills to contend with. Our vote would definitely be for 38 if that would make sense for you, but even at that 42S/42 wasn't bad. Don't know about the stretch of 42 from Coos Bay to Coquille. Google Maps couldn't find Humbert Mountain, OR, so I don't really know where you're starting from.
The North Umpqua Highway 138 is a great drive over to 97 - a nice 2- and 4-lane highway, no issues with a trailer, and as I recall, the waterfall parking areas generally tended to have enough parking for trailers. Crater Lake is just a short drive out of the way, and the Newberry National Volcanic Monument is right off of 97 to the north - highly recommended. And off of 97 north of Redmond just outside of Terrebonne don't miss Smith Rock State Park.
Discretion is the better part of boredom.
Tow Vehicle: 2012 Toyota 4Runner Trail, KDSS, BFG A/T KO's, ScanGauge, B&M Tranny Cooler, 20" Opt7 LED Light Bar, LED DRL's, Prodigy P2 Controller, Timbren Rear Springs