Sounds like the audio amplifier in the unit went. If you are using RCA out to the TV and you have to turn it all the way up, and the regular speakers do not it function. You are, more likely than not, only getting pre amp output.
Now thoughts are that bouncing down the road cracked or broke a solder joint in the unit. At which point changes in moisture or temperature may let it work occasionally.
One more thought, sorry for asking, the mute is not on, is it? Newer units you can still hear a very little sound in mute.
2015 F350 Lariat Diesel Dually, White, Hitch Kit.
2013 Dutchman Voltage 3200 Epic II 5th wheel.