Most new RVs come with a hefty inverter. The one in our 2017 Roadtrek CS Adventurous XL 4X4 puts out 2,500 watts! That's great for the microwave and the air conditioner. But the rest if the time, I don't need nearly that much power to charge by cellphone, laptop, tablet or camera batteries.
My friend the famous Campskunk joins me in this video to show how to add a second, smaller inverter to your RV.
The inverter is the Wagan Elite 400 watt pure sine wave. I got mine from Amazon at http://amzn.to/2DgqiWB
Circuit protection is your friend! Make sure to be safe and always install any DIY project to proper specifications.