What family_wagon says above is true... Here's how to decide.
You have four 15A breakers. If you have correctly sized your system so you can handle the full rated load everywhere, then you want to change the main breaker to a common-trip. This is because the plastic handle between those two levers right now will make them SWITCH together, but not TRIP together. However, a common-trip plate (or a breaker designed to do this internally, which will be fat like this but only have a single switch - like the main breaker in your house)
However, if family_wagon has guessed correctly and you're only planning a 30A feed, you should DEFINITELY do as described above - bond the two power lugs and switch this out for a single breaker.
If you're in the U.S., you're also supposed to put a label on this that says "Service Disconnect" to distinguish it from any other kind of breaker.
FYI, a common misunderstanding/myth about breakers is that they're designed to protect people. They're not - at least not from electrocution. Humans can be electrocuted way before a breaker would trip. GFCIs will do that, but breakers are designed specifically to protect WIRES - you size them to trip before the wire is in danger of overheating and causing a fire.
I just wanted to call that out for posterity's sake, and it may be relevant to your decision. For example, if your site hookup cord is only rated for 30A, then family_wagon wins this round - you must change the breaker. Actually, technically, this would be over-rated even for a 50A cable, although site hookups and many generators will trip at 50A anyway. Most main breakers are used as service disconnects, not purely protective mechanisms.
I am not an electrician, FYI - and I don't speak for him but I'm guessing family_wagon isn't either. We're trying to provide some helpful advice but ultimately you're responsible for a safe installation here. I would definitely recommend consulting an electrician if you have ANY questions. As long as you're OK with a verbal instead of written sign-off, a lot of them will have a look just for a half-hour's labor fee and maybe a 6-pack (or a tour). Call around - if you can find somebody who'll do it, especially for the cost of what you already spent on your wiring... It's money WELL spent.