I too have looked into doing this. I spoke to a Class A driver instructor, he in turn asked the CDL officer for CHP. Here's what I got back. Pulling a trailer behind a fifth wheel is legal, behind a travel trailer is not. You must be under 65 feet total length for California. Some of the other states are 60 feet, so you may get in trouble if you are 65 feet and you go to that other state with a 60 limit.
As for the license, you will definitely need a class A, either a commerical class A or a non commercial class A. You will also need the doubles and tripples endorsement for that license. When you take the written test, you will take the non commercial class A and the doubles and tripples. You will also need to take a driving test, I was told that I would need just the truck and the fifth wheel for that, I would not need the 2nd trailer for that one. I already have a commercial class B so I have expericenced the next bit of information he told me first hand. He said that you will should go to a DMW office that routinely deals with commerical drivers and tests as they will know the requirements far better than one that doesn't. Also, for more information, I would find a CHP officer who specializes in this subject and do more research.
One other thing about california and driver's tests, when I got my class b, it was for driving school buses, the district that I worked for only had automatics. As a result, I had to test in an automatic vehicle. My license has a restriction for automatic transmission for commerical vehicles. If I had tested in a manual, I would not have this. My point is that if you have an auto now and get your license, then later get a stick, it could be a problem. My truck is a stick, so I plan to test with it and should be good to go.