I bought a Garmin RV 770, specifically for use in my RV (duh). I bought it from Amazon well before actually having the RV, so returning it isn't an option.

When I was driving the RV home from PA, I noticed the display was dim. When I tried to increase the brightness, the unit said "You must use the factory supplied power cable to go over 40% brightness" or somesuch. Problem is, I AM using the factory supplied power cable.

Now I will admit to this....I have two things using 12v power; the Garmin and a tire pressure monitoring system. I bought a dual 12v outlet at Walmart, installed it under the dash, and tied into the power cable that supplies the 12v outlet in the center console.

My first thought was maybe since I had two devices plugged into a wire that was only originally supplying one outlet with juice, somehow it wasn't supplying the Garmin with the proper amount.

Any thoughts?

Are you using the supplied cigarette lighter power cable, or the supplied USB cable with a 12V USB adapter?

If you wired the outlet correctly, it should be able to supply way plenty of juice for what you have plugged into it.

The 12v cigarette lighter power cable.

Do you have a multimeter you can use to measure the 12V outlet voltage?

Garmin has pretty good support. Can't hurt to call them to see if they can help.

Try to be near the unit in the RV when you call.

(I've owned quite a few Garmin products, have called or emailed support at least six times over the years and have always had good luck with them.)

You want to make sure you have the required amount of voltage and a good ground on your supply plug at the dash.
What I did for my gps and tire monitor is run a separate circuit using 12ga wire for the power and ground wire, and a plug under my dash and plugged both into the plug using a Y adaptor. I have been plugging my dash cam into my dash plug.