If you plan on towing a 5th wheel, a diesel truck is definitely the way to go. You could get by with a gas engine, but it will provide a less than optimal towing experiences. The torque curve for a gas engine is such that it makes peak torque at much higher RPMs than a diesel, so in other words, the diesel will get the job done without a big fuss, with considerably less wear on the engine and improved MPGs over gas engine.
As far as diesel trucks are concerned, it all depends on your price range. You said that you live in CA, so any diesel model year <1998 is smog exempt. If you are on a tight budget, take a look at pre-1998 Ford's or Dodges. Dodge had the Cummins 12v which is a proven engine and will last a long time. Ford began offering the 7.3 powerstroke in 1995 which also has an excellent reputation. Before that Ford had the 7.3/6.9IDI engines which also are good power plants and don't have any fancy electronics, so are easy and cheap to fix. 1994 was the first year that Ford offered a turbo, so anything older than that might not have one unless it is aftermarket. The Chevy/GMC diesels of that era were the 6.2 and 6.5, which are not known for being especially suited for heavy towing, they were more intended as engines to achieve reasonably good mpgs in place of a gas v8. Depending on who you talk to, some would say they weren't too reliable.
For some of the newer trucks, Ford offered the 7.3 powerstroke until 2002/20003 after that, there was the 6.0 and 6.4 powerstrokes which don't have a good reputation, especially the 6.0l. Avoid those if possible. The duramax overall has a good reputation, they had some issues with injectors in the early 2000's. Dodge had both the 5.9 cummins and about 2005 or so switched over to the 6.7 cummins. Both are good engines and I don't hear much bad about either of those.
As far as price range, you can get an older Ford or Dodge for <$10k, early 2000's trucks will be in the $10-20k range and the newer you get will be closer to the $50-60k prices of the current ones. Hope this helps you out.