Your hypothesis is prob correct. If it was left on when you stored it and your battery was connected it likely ran battery down over winter and when you plugged it in it was cool enough inside to activate the thermostat. Of course, if you did not have a battery attached during winter the same could have happened. Your camper was stored at a warmer temp when you stored it. Good idea to turn off the system at the thermostat rather than using the temp setting only. Especially while storing or in transit. Also you should shut propane bottles off when storing or between trips, disconnect battery also.
No worries though, you've probably not damaged anything.
2014 Windjammer 3025W Diamond 34.5 ft 8300 lbs.
2010 F150 SC XLT 4x4 5.4L, 3.55 gears 20" rims
12 k Equalizer WDH, Firestone Airbags, HD Bilsteins
TST 507RV TPMS