I think 'Boatycall' has zeroed in on the probables.
Get an inexpensive outlet tester at HOME DEPOT or LOWES. One with a GFCI test button might be a more useful tool in the long run.
Stick this 'puppy' into any 120 volt outlet in your camper and it should quickly verify if the conditions that 'Boatycall' indicated are the problem. But FIRST, test your camper's outside power source ! I _always_ test the campground's pylon before I hook up to make sure I know what I'm dealing with ( you can get an inexpensive 20amp to 30amp plug-in adapter at any RV supply store or WALMART ).
Once you determine that you have a condition described by 'Boatycall', then, if you are not so comfortable handling electricity, you can decamp to any RV dealer who has a certified electrician who can track down the cause.
Because an RV is not usually grounded like a house, the issue could be in the main or any branch circuit. From where the power enters your camper to any fixture or outlet, you could have a polarity reversal pretty much anywhere. A multimeter and a careful process of elimination is necessary to zero in on the culprit.