yes very normal..when you are hooked up to shore power the converter is charging your batteries AND all your 12 volt stuff (lights etc.) so it is normal for the fan to run. If it continues to run after the batteries are all charged up and little or no 12 volt devices are in use, then the fan relay may be stuck. If your batteries are way down, which they may be it may take 8 or so hours to bring them back up.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME