Did you plug those into Google Maps to see the distances? Google Maps gave me these preferred routes:
1780 miles - I-80 to I-76 to Denver I-70 to I-15 to I-15 in Utah. I-15 south to St. George to I-215 in California
1786 miles - I-35 to Kansas City, I-335 to I-40 west to Holbrook, AZ, 377/260 to Payson, AZ; 87 south to Phoenix to I-10 to 85 to Gila Bend, AZ then I-8 to San Diego.
Of these 2 the most scenic is #1 - I-70 from Denver but you'll have some big mountains to cross.
The second route is a lot easier driving and scenic from Holbrook AZ to Phoenix. If you're having radiator problems this would be the route to take for less mountains but it's going to be hot temps. You might want to cross from Phoenix to San Diego super early in the morning... like leave at 4am. It's even hot at 10pm so morning would be best to cross the deserts unless you want to drive through the night. Carry any maintenance supplies you may need for the RV and plenty of water.
Your 3 choices:
#1 you'll have a biggie.... Donner Pass into Sacramento & an assortment of smaller hills along the way. It would be the coolest temps overall than any of the others.
I-70 is more scenic across Colorado than I-80 across Wyoming but I-70 has some big mountains.
If taking I-70 across Colorado it makes sense to take the I-76 diagonal to Denver.
All those big cities don't appeal to me. I'm used to weaving around via secondary highways.
(You might want to create a Signature Line so we can see what you're driving. It helps in replies.)