A 15 amp outlet is not usually advised to run an air-conditioner from, especially a 15,000 BTU one. If you will look inside your RV's electrical distribution panel, you will most likely find that your air conditioner has it's own 20 amp circuit breaker.
Also not knowing exactly if you are using any kind of undersized extension cords, you may have done some damage to either the cord and or A/C as you are most likely overloading something by trying to run your A/C from a 15 amp house outlet..
Please read the following article by Mark Polk, where he touches on the situation you are encountering.
BTW, welcome to the FRF family.
Basic RV Electricity - RV Information (RV Maintenance)
and then this one explaining converter draw: