Cheapest way to go from San Francisco to Los Angeles?

Almost certainly the bus + hostel option will be cheapest, however don't rule out the possibility of flying - it won't be cheaper, but it will save you a lot of time.

SFO/LAX/LAS are highly competitive routes for airlines, and it's not uncommon to find very cheap fares, especially if you can be a bit flexible in terms of which day you're traveling. For example, in the next few weeks I can see $55 flights from SFO to LAX, and $47 from LAX to LAS! You'll need to take into account extras such as baggage fees.

The other option is to catch the train. Amtrak has routes between these cities, and especially for San Francisco/Los Angeles the prices can be similar to those of the buses.

I've actually done this (albeit about a decade ago). If you book your tickets more than a week in advance, you can get hefty discounts on Greyhound. These days there are also several other cheap bus companies around the US (Megabus, Boltbus and their ilk) and occasionally the train works out well.

In Vegas, however, consider upgrading to a hotel on the strip. If you can time your visit during the week, hotel room prices halve, and we had an incredible room overlooking the Strip. But otherwise, hostel for sure :)

Another way to save cost - which is what we did - is to travel at night by bus. We didn't sleep very well on the bus, but it saved us some cash.

And the Green Tortoise in San Fran is meant to be one of the best hostels in the world (haven't tried it myself) - I'll definitely be trying it out next time I visit there.

I would say #3 would be the cheapest. I doubt that a RV would be cheaper for such a trip than a Greyhound bus + motel. RV's are better for long distance long term trip, or traveling to National Parks etc where motels are harder to find and no public transportation is available.