You didn't ay, but I am assuming your RV is plugged into a standard 15-Amp or maybe a 20-AMp exterior receptacle at your friend's house.
And, remember, at times, he may be using the same AC-Voltage circuit you are trying to use.
If this is the case, you are just trying to use more current than they have available for you, from their house.
Your electric Hot Water Heater, and then your Coffee Pot and then your Microwave draw the most current of the commonly used appliances in your RV.
If you're going to be there for a year or so, then you should get a standard 30-AMP service wired into your friend's house. Or, learn to use only one of these appliances at a time, every day.