It is REALLY easy to forget that the 110v is off.
Yesterday, I had to replace the ac thermostat in my RV. Turned off the 110v and disconnected the battery so I couldn't short wires and damage anything. Replaced the thermostat, turned on the battery, tested the furnace. Worked like a charm.
Tested the ac. No joy. Fan would not even come on.
Turned the battery back off, took the thermostat back off the wall, went in the house to get VOM. Coming back out, I looked at the pedestal and had a DUH moment.
Put the thermostat BACK on the wall. Turned the battery BACK on. Turned on the 110v. The ac and its fan worked like a champ.