I have spent quite a bit of time on the aluminum flat deck conundrum. If you want a deck that is the exact size of your camper like most people, you have to build your camper to a flat deck manufacturers sizes or have a deck fabricated to the exact size you want your camper to be.
I didn't want the size of my camper to be decided by the size of the decks that are on the market like the "Ute Deck" used by FWC. So I decided to have a deck custom fabricated to the dimensions of my camper.
I would believe you have many qualified aluminum fabricators in the LA area that can make up an aluminum deck if you chose that route.
If you prefer a prebuilt one, have a look at Aluma. They make nice sized ones for full sized trucks like yours. Their 8100 series can be had in 81" wide by 7 and 8 feet long, should cost about $2000 and weigh under 300 lbs. They list 5 dealers in the LA area. Bakersfield, Ridgecrest, Ventura, Beaumont and Ramona.