Generator Help

No, you cannot use the 240 volt l6-30R receptacle. It has no neutral.
You could use the 5-15R, they are rated 23 amps.

That's a rather strangely wired generator, from what I can tell. I think it has two independent 120V outputs, and the switch presumably either puts them (apparently paralleled and going through the 46A breaker) to the 120V outlets or couples them in series to the 240V twist-lock outlet (protected by the 23A breaker). If that's the case, there's nothing (other than the stickers) preventing you from drawing 40+ amps from one of the 120V 15/20A sockets.

If that is indeed the wiring, it should be possible without too much work to wire it with a 120/240 outlet of some sort and connect a 50A RV directly. (This would be a four wire socket of one of the appropriate types, be it twist-lock or whatever.) I just wonder why they didn't do that in the first place and have the generator be permanently 120/240V, which seems to be the standard way of doing things.

Maybe the internal construction is quite different than I'm guessing, and the voltage selector switch is doing something like setting the voltage regulator or field voltage or something else really weird.

Edit: Just to be clear, you absolutely cannot connect a 50A RV to the existing 240V socket and expect things to work and remain undamaged.

You can not use the L6-30. It would supply only 240v and likely burn up things in the trailer. You must have a neutral and it does not.

What the hell?

That Onan has a CENTER TAPPED 240VOLT STATOR!

EACH 120 volt receptacle uses an OUT OF PHASE HOT. L1 and L2. L0 NEUTRAL is the third slot in that 240vac receptacle. Generators from 100 watts to 100 megawatts DO NOT USE A GROUND. GROUND IS EARTH. TERRA FIRMA. Where do you think that third terminal connectuon goes? Mars? It goes directly to the center tap of the stator.

PROVE IT WITH A VOLT METER
The voltmeter proofing should have been the first and only answer many of you should have given.

E=MC2 assumptions always make me grumpy. OK sue me - grumpier.

Here's a guy who needs a generator and was getting his arm twisted that it could't work.

It helps to have had something apart and physically see how it works then check it with instruments. It and a few several dozen like it.

STRUCTURES may have 3-prong connectors for stoves and dryers that have two phases and an earth ground. But generators have neutral in receptacle connections. EARTH GROUND CONNECTORS are usually an aluminum block bolted to the machine's frame that has a hole and set screw. A wire is attached from there to a stake driven into the ground.

The photo doesn't look like an RV genset. It could be a construction site unit. The L6-30R does not have a neutral.
OP: could you locate the exact model of genset and post it?

See page 28 for wiring diagram
Manual