Here's a little more detail on the three choices. Going Hwy 20 will be difficult in a day. Simply a slower route, I would allow two days for that journey. Stopping for a overnight in Ancient Redwoods RV park along Avenue of the Giants. The temperature will start to cool down around Avenue of the Giants. Once past Scotia you can usually turn off the A/C.
The Redding route is the most challenging drive of the three. Hwy 44 from Old Station to Palo Cedro has sections where you will need to keep your speed in check (curvy, narrow, and downhill) from Palo Cedro to Redding its 4 lane freeway. Redding is a bottle neck, but in 10 minutes you'll be through it. Once past Whiskeytown lake you will start hitting construction delays. As you drop down 299 you will probably see fog. Soon you can turn off the A/C. Once on 101 you will make pretty good time to Crescent City. A lot of this stretch runs along the ocean, and probably the prettiest drive (and coolest) of the whole trip.
The third route is a easier drive as it takes hwy 89 into Mt Shasta City instead of going down 44 to Redding. However, once you leave Yreka I-5 will climb into Oregon and keep climbing for several miles. Then you get a long downhill into Ashland. after that its a piece of cake. On 199 you have to go through Grants Pass, but a good road until you get to the other end. There is a section where the road is very narrow, and curvy, as you get near Crescent City. This is the fastest route. The only towns you slow down for are Susanville and Grants Pass. This is still a long drive (420 miles) in a RV. If you need to do it in a day, leave earlier.
Where you are going after Crescent City could influence which route I would take to get there. Be safe and enjoy.