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Could you answer a few questions?
1. Are you parked in a campground or is this a 50A outlet installed at your home?
2. You stated you had 240v on a 120v outlet, was this actually measured with
a meter?
3. What surge protector do you have? Most of them will detect an open neutral.
4. What electrical appliances/loads were running when this anomaly occurred?
By code, a neutral is grounded at the service entrance point wherever you are and is supposed to remain solid throughout any distribution circuits. This is the point in the system that offers the lowest impedance connection and will be common with the entire system. By symptoms described this certainly sounds like an open or high impedance neutral. There should be a good neutral connection at the pedestal you're plugged into. You can't really establish a neutral in the RV, it's possible to lose it in the RV.
You stated that you could unplug your cord and plug it back in and the problem would clear for a period of time. You later stated when you plugged your friend's cord in, it cleared. If you didn't leave your friends cord plugged in for a while, I'm not sure you really proved anything if you didn't leave it for a time.
I think the 50A outlet on the pedestal would be the best place to start, use your meter and confirm you have a good neutral there. Those 50A male plugs take a substantial amount of force to plug in and if the receptacle is not mounted securely you might be moving it enough to cause a poor connection to get worse or even open. If you're convinced it's your power cord, you can buy either or both the male plug or twist-lock female separately and would be much more affordable than a whole new cord. Unless the cord has visible damage, it's likely good. The female twist-lock power inlet on your RV is another likely suspect.
Open neutrals are often a real head scratcher, they can manifest a long way from the source of the problem; if they are intermittent it's even harder to find. For future reference if you're using a digital multimeter, they can at time lie to you, even high end expensive models. These meters offer a very low impedance and are just not accurate sometimes; you need to put some load (resistance) across the circuit. In an instance like measuring voltage between the ground and neutral bus in your RV panel, an old fashioned pigtail light socket with an incandescent (not LED) light bulb makes a really good tester. Get a rough service coated bulb for safety if it should shatter.
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