Anywhere you can access a pair of wires. Just remember that Airstream ground wires are white and the ground wires on most, if not all, outlets are black. Polarity Can matter!
I added 2 in the bedroom, one in the bath, one in the galley, and one in the outside refrigerator compartment. I plug a fridge fan into that last one. From the outside outlet, I can also reach the tires on both sides with an inflator.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10