Think about two 6v batteries wired in series... B1 negative is connected to B2 positive. So if you measure the voltage from B1 positive to B2 negative, you get 12v. But B1 positive to B1 negative is only 6v. That connection between B1 negative to B2 positive would be considered the neutral wire in an alternating current (AC) system. You don't want to touch B1 positive to B2 negative because you'll get a big spark!

So its like a neutral.

Really? So if you attach the connecting wire between the two 6V batteries in your camper to the system ground, just like A/C does at the breaker box, all will be well? Please do this in a safe place should you not know the answer. And have a fuse in your test wire.
There are fundamental differences between A/C and DC, and that can make some comparisons difficult.( Like saying cars and trains both have wheels, and both turn corners, so they must steer somewhat similar.).
For anyone else: this borders on a dangerous bit of misinformation. That connector wire, UNLIKE a neutral, is NOT a good ground or current return. Never use it as such. Ever wonder why electric golf carts DO NOT ground the battery pack to the frame? There is good reason.
Please elaborate on "out of phase" connections.
well...you probably wont have two phases in your residential house power anyway...
In the link to Wikipedia, please note this critical line:
""Three-wire single-phase systems, with a single center-tapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America.
This arrangement is sometimes incorrectly referred to as "two phase".""
If you accidently 'swap' your hot and neutral in the 110V 30A RV receptacle wiring, the connection between the neutral and ground in the camper should trip the supply breaker to the outlet instantly on plugging it in. Maybe burn the plug and/or receptacle contacts a little too. But it errs on the side of safety.
I can see a scenario where you could have a 50A, 4 lug outlet mis-wired. ESPECIALLY when 'upgrading' an existing 240V 50A 3 wire outlet to a 4 wire receptacle so you can use common adapters for your RV. On a 50A shore power cord, the neutral/ground connection inside should trip the outlet supply breaker. Again, with possible arcing and damage during insertion. With the 'splitter' or adapter, it could give you a working camper, but 120V between the step outside and the ground below.

Get a severe 'tingle' when touching a metal part on the camper while standing on the ground outside. Or worse.... 240V instead of 120V blowing up devices. AND a 'tingly' camper when touched.
This thread has gotten into a potentially dangerous area,
PLEASE... DO NOT TAKE THIS INFORMAL DISCUSSION AS QUALIFIED INSTRUCTION. You are dealing with 'invisible' forces that can kill you or others with little or no warning. Electrical code,
and inspectors, are there to ensure your safety. The deaths and injuries of others before us has led to the code we have today. Dont join them by ignoring code.