The problem is at the house, not your run to the RV receptacle. It's low because it is leaving low when the AC is on. I have a list of questions to try to get a picture of what you have. You may ask the power company to check your drop wire that all connections are good. With the AC on and you get a voltage drop say from 120 to 110, they may say that it is within tolerance. Power companies will always respond to check there side of the meter. It has happened with an overhead drop wire that one of the connectors has corroded causing a bad connection, thus you'll get a low voltage condition with a load. I'm assuming that the panel is at your house.
- The 6/3, is it two hots, a neutral and a ground? Or one hot, a neutral and a ground? 6/3 means four wires total, 6/2 means three wires total.
- Is it one cable or three separate wires in conduit?
- What size at the panel is the main breaker? 100 amp or 150 amp?
- What is the age of the drop wire from the power company?
- What is the voltage at the panel and at the RV connector without the AC running?
- Have you tried it with a electric dryer running and the AC is off.
- How old is the AC and it's size? Old units take a lot more power.
- Anything running at the RV when you check the voltage?
- When the AC first turns on, do any lights dim inside your home? (incandescent filament bulbs)
To use this calculator [
http://wiresizecalculator.net/calculators/voltagedrop.htm] enter the expected maximum current that you will draw. It should not be 30 amps. Maximum continuous load should not be more than 24 amps. If you are leaving the panel at 120 volts, at 24 amps, it should be around 116 volts. If you leave the panel at 110 volts with a 5 amp draw it would be less than a volt drop. With no load at the RV, the voltage will the same at both ends. Voltmeters take nearly no power. The RV may recommend a 30 amp breaker to prevent false tripping when AC motors are turned on. On start up is when they take large slugs of current that could trip say a 15 amp breaker.
Question is would it fix my issue if I changed the breaker to a 50 amp and put a 4 circuit panel box where my RV plug is then reduced it down to a 30 amp 120v circuit? Increasing the size of the breaker will not change anything. The breaker protects the wiring, not the RV. Doesn't most RV's have there own breakers of fuses? I really do not know. A lower voltage at the RV will affect AC motors. If there are any DC motors then the RV is using a converter and low voltage input should not affect it. Reach4 does have a good suggestion.
http://wiresizecalculator.net/calculators/voltagedrop.htm