Dead battery comes to mind. Since it sounds like you are on shore power, it may be your converter is not working correctly to charge the battery. That would be the first thing to check.
Your lights run off battery but your outlets use the shore power, the noise you heard may be the co dectector, mine goes off when I have low battery.
HTT: "EscapeII" 2016 Shamrock 23WS (current)
PUP: "Escape" 2010 Rockwood HW 277 (gone)
TV: "Bruce" 2011 Dodge Durango V8 Hemi Crew
Just us gals (me, Sis and our daughters)
Traveled 19520 miles and 209 nights in 7 years and counting