San Diego thru Laguna Hills to Fresno, Ca.

There's a Pilot in Castaic and a Flying J in Gorman/Frazier Park that's pretty easy to get in and out of. Plenty of space around the Flying J to park if you want to stretch and rest awhile.
Travel Authority and Petro on the north side of the grapevine before the I5/Sr-99 split.

From Laguna Hills head north on I-5.
In Anaheim, head north on CA 57 to CA 210 West.
This will bypass LA completely. CA 210 rejoins I-5 north of LA.

If you're looking for a rest stop, the State rest area at Lebec is now open. Huge area with a free dump station. There is a Flying J station in Frazier Park.

As for camping..IMHO, there aren't any decent campgrounds north of LA..I would go up and over the Grapevine and stay at the Bakersfield RV Resort or the Orangegrove RV Park. BRVR is closest as OrgGrove is out CA58 towards Tehachapi. Both are excellent campgrounds.....Dennis

Laguna to the top of the valley: Personally I would take the 405

Even though its only slightly shorter, the I-5 is crawling with trucks 24/7 in both directions and there are WAY more accidents and jams on the 5 than the 405. 405 has construction in a few places as well, but it's a way easier drive.

While Dennis's route MAY have less traffic (one never knows around here), it's considerably longer.

Timing: Leaving San Diego at 8AM will put you in Laguna around 9:30'ish. A 2hr layover in Laguna will put you on any route you choose around noon-1:00, and that is the peak of Sat traffic, which is often as bad or worse than rush hour because it can encumber much longer distances. Sunday morning is about the only really light traffic time from that direction.

Welcome to LA

I would take Hwy 15 north to Hwy 58 and then head west to Hwy 99 and north to Fresno. Much less stressful than going through the Los Angeles basin which starts to get ugly as far south as Capistrano, or to have to go over the grapevine on I-5.

There is a very nice RV park at the glider airport in Tehachapi which makes a good stopping point along this route.

I drive to and from San Joaquin Valley to San Diego several times a year. If I am stopping in Orange County I like the route posted by Executive. I know Jeff said is considerably longer but it really is only slightly longer and generally even with traffic is faster than either 5 or 405. Both 5 and 405 are fine if traffic is moving but that is really hit and miss. Middle of the day on a Saturday is tough to predict. One little fender bender on any of these routes and you have stop and go traffic.

If you don't need to stop in Laguna Hills a route like Redsky posted that bypasses both OC and LA works well.

Besides what has been posted for stops if coming over 5 you can also stop at bottom of grapevine at Tejon around Lebec Road.

While longer, I'll yield to Dennis on this one since the OP is attempting it on a Saturday afternoon. A total crapshoot.

The actual mileage difference in my opinion is minimal. 5 is the shortest, 405 adds 8 miles over 5 and 210 adds 16 miles over 5. It is always a crapshoot anywhere from San Clemente to Santa Clarita.