First Night or Two out of LA

Lancaster or Palmdale (Acton, CA)

Just south on Interstate 5 from Santa Fe Springs is Anaheim Resort RV park. Cal-Trans is widening the freeway in SFS and it is a mess, but if you're heading south, you'll be out of it soon. There are a couple of Wal-Marts within a 15 minute drive. If you're bringing kids, you can watch the D-Land fireworks or catch a shuttle to Downtown Disney. It doesn't cost to walk around there.
If you want to get a little further out of the immediate LA area, off Interstate 10 in the Pomona/San Dimas/La Verne area, there are two private RV parks- East Shore RV Park in Bonelli Regional Park and the KOA Fairplex on the grounds of the LA County Fair. Stores are nearby. (For safety, I would recommend shopping only in La Verne or San Dimas. Parts of Pomona south of I-10 are sketchy.)
Further east on I-10 near Palm Springs is Beaumont and Banning. There is a KOA there and you can see Wal-Mart off the freeway. This may be too far east for your travels.
If you are heading north on Interstate 15 to pick up Interstate 40 to the Grand Canyon, try to get over the Cajon Pass into the high desert towns of Victorville, Barstow, Apple Valley. There are shopping malls all along the freeway there. Mojave Narrows Regional Park (San Bernardino County Parks) has a campground, but I have not been there.

Pechanga Casino/RV Park in Temecula is awfully nice for what it costs. Lots to do and every big box store you can think of is within a few miles.

depending on the weather (right now it is unseasonably hot) I would go to Desert Hot Springs or Palm Springs, or Joshua Tree NP.

closer in? Rancho Jurupa Park, which is a county park in Riverside. Then spend the day exploring the Old Mission Inn.

Indian Waters in Indio.

You need to be aware of the traffic you're up against. My suggestion is that you first head to the closet RV Park near you, probably Anahiem RV Resort, and then time your departure from there around commuter traffic. If you are heading to the Grand Canyon first, Desert Willow In Hesperia just off 15 is much better than Mojave Narrows because of the comparative distances off the freeway. Next stop I'd recommend is Blake Ranch just east of Kingman. Very easy shot to the Grand Canyon from there.
The drive from LA to Pechanga, which is a very nice park, would be horrific during commuter hours.

Thanks for the comments, re LA Freeways is there any time during daylight hours when it's quieter? how is I-5 down towards Oceanside mid afternoon say 2-4pm

Check out google maps, click on traffic - from there you can chose to see what average traffic is like on any day/time

chrisaussie wrote:
Thanks for the comments, re LA Freeways is there any time during daylight hours when it's quieter? how is I-5 down towards Oceanside mid afternoon say 2-4pm

I'd veto the Oceanside option. The only advantage of going that way would be to cut across to the I-15 for the trip North.

I second the vote for Desert Willow in Hesperia. It gets you away from most of the pain of Socal and sets up a nice next leg of the trip. We stayed there back in September when we we needed to visit Hemet. That way we kept the TT up the hill at the CG and took our truck down for the day visit.

There is a Target and a Walmart close by.