We rode 139 from K-falls to Susanville in July on our motorcycles and I don't recall any thing that might be an issue for a MH. I wasn't thinking of MHs at the time but now we are. Check out The Boomer Zoomers - Reno to Klamath Falls
The pavement is good (sections of lots of tar snakes - tar crack sealing - not a MH issue but can be a motorcycle). As I recall the lines are "standard" width but not on the wide side.
Moderate ups and downs with a rather good descent into Susanville coming in from the North. You can "drive" the route with Google Earth and see what it looks like - I just did.
Stop in Adin at The Oney diner. Fresh cooked food (slow compared to Micky Ds but one h*l of a lot better) and it beats cooking yourself - at least it was for us!
We rode to the West Tule Lake and stopped at the visitors center in the Lava Beds National Monument. Might be nice diversion/stop if you've not been there. On the bikes we left Reno about 9am and got into K-falls about 5pm. We ride about 5 to 10 over the speed limit unless the CHP is around.