I drove CA-36 last week from Red Bluff to Eureka.
There is no fuel between Red Bluff and Hydesville, except for one station near Platina. It was $1 per gallon for gasoline higher than Red Bluff.
The mountains will cut your fuel mileage to about 1/2 of what you normally get. Being from Arkansas I didn't find CA-36 ususually challenging, just a bit longer grades both up and down. Some steep grades. But then again I though driving up CA-78 from the Salton Sea to Julian while pulling the trailer was a nice drive even though it was foggy and raining.
I would not recommend CA-36 for someone uncomfortable towing on mountain two lane roads with no guard rail.
There are many areas where the road is under repair, but on a good clear day, there were no traffic delays.
CA-299 is the preferred route across the mountains for local folks on the coast. There is reasonably priced fuel at Weaverville - so the longest run between fuel stops is about 100 miles. CA-299 doesn't reach as high as CA-36, so the grades steepness and length is a bit less. So you will likely get about 60-65% of your normal fuel mileage. Again - take it slow, and don't floor it going up grades.
Another route across the mountains is US-199 from Crescent City to Grants Pass, OR. That is 80 miles between Gas Buddy reported stations
FYI folks - Gas Buddy is not worth a darn when a person gets into the mountains, and places far from the main cities.