We have driven 199 to Interstate 5 many times. It is a good road and a few places along a portion of the road from Crescent City through Patrick Creek (maybe beyond) some of the winding road is a little tight... but with a 20 ft. trailer you should have no problem. It is not, however, a fast road because it is curvy on a good deal of the way. The road is a good one, however, with good pavement. Along 199 you will pass by the turn to Oregon Caves.... worth a stop! You will have to likely unhook your trailer for the last mile or so, but you could call ahead and see if there is enough parking for your trailer and rig for the visit. (No camping there, but a sweet old lodge).
Instead of going along 199 you could travel up the Oregon Coast to Reedsport (it would take an extra day probably) as there are some wonderful sites and campgrounds along the way. Oregon State Campgrounds are generally very nice! From Reedsport you can travel along Hwy 38 to the burg of Drain and then on to Interstate 5. Cottage Grove just north of the area from Hwy 38 has a couple of nice campgrounds. If you head up to Salem I would take an extra day or two and drive east to Silver Falls State Park in Oregon. Fabulous place. But otherwise head north and south of Portland is Champoeg State Park another nice place to stay (a bit off the road, but nice!).
Have fun on your journey and post a few photos! we love photos!