Unsafe outlet?

Is this in your RV or a nearby outlet? If the black and white are reversed and you plug in something with a case ground it could blow the fuse. If you have to ask I guess you shouldn't. That said, I have a 2 and ground (blk, white, and ground) underground line that runs for 500 ft to my 220 vac ditch pump. Once a year I need 110 in that outlet for an air compressor to blow out the lines. I use one hot and a ground to my compressor. However, I know what the dangers are and careful and know how it is wired. Probably should have put in a 3 and ground but too late now. For 15 minutes a year I am not going to put in another 110 volt line. JMHO.

If you have to ask, then I have to say, no, it is not safe to use.

Ditto - don't use it. Some ungrounded devices use neutral for the case. That's why the neutral blade is wider on those type things - to keep you from doing what you are asking. I suppose if you have some ungrounded thing with both blades the same width it wouldn't matter, but why take a chance.

That by itself is one thing, it is a question of "what else may be wrong". Best to have it fixed.

Yes, you could use it for a short period.
Is your rig equipped with 30 amp shore power or 20 amp?
If 30 amp, are you connecting to a location that has both and are you testing the 15/20 amp receptacle and not the 30/ You could use your Klein tester with 30 amp adapter to test the 30 amp.