Most direct route is I5 south to just below Eugene, Or. From there depends on where you plan to stay in or near the park. If you do not plan to take the MH to the lake rim, I would not use the parks north entrance. If you prefer I5, take it to Central Point and then follow signs to Crater Lake which will bring you to the south entrance which is where the Mazama campground. Or, can catch hwy 58 just past Eugene to hwy 97 to Chiloquin and then cross over to hwy 62 and back up to the south entrance to the park. I have done all three routes in MH towing our edge without difficulty.
There are some long climbs no matter which route you take. I grew up and lived in Fort Klamath in the 50's and 60's and worked for the park service as a truck driver in the summer. Gorgeous place, I always tried to arrange to eat lunch somewhere around the rim. If you are capable of hiking down to the lake, take the boat ride if they are still operating. You see things from lake level looking up that you can't appreciate from the rim drive although you will certainly want to do that as well.
Have a great time.
Gerry & Louise
2007 Tiffin Phaeton 36 QSH
2018 Ford Edge Titanium