Sounds like your contacts are stuck in the generator position. That transfer switch is energized when the generator is powered up, and de-energized to shore power when the generator is turned off.
Slap the front of the transfer switch box with an open hand, and see if it drops out.
2005 Georgetown 342DS XL, Full Banks Package
2005 Liberty Renegade 4X Toad
1989 Wrangler Sahara Toad
1984 19.5' Beachcraft i/o 5.o liter, 4-bbl (and you thought your Moho sucked gas!)
D/W Carole and Maggie the Teacup Lab (Black Chihuahua)