I would take I-5 to Grants Pass, 199 (or 299, I can't remember)to Crescent City and 101 to Redwood Park. I have driven my 36' MH with no trouble. Now, to get to Lassen does require a windy, serpent road. 299 is just that between Eureka and Redding. It is doeable, but if you are worried about the curves, you need to think about it. The only other way is to take I-5 from Grants Pass. But, you still have to get to the coast for Redwood Park.
Personally, I would go over to the coast from Grants Pass, and then to Redding and on to Lassen. There is a KOA CG in Shingletown, which is just a short drive to Lassen. Only a couple of restaurants there though.