A little water in the bottom of tank is OK.
I remove anode, let the tank drain then re-insert the rod finger tight.
I have being leaving the last bit of water in the tank for eons and never had a problem here in the great white north where temps get down to -40 at times.
2007 Surveyor SV230 - 200 Watts Solar/MPPT Controller - 230 AH Battery Bank - 600 watt PSW Inverter - (2) 2000 watt Inverter Generators - LED Lighting - Boon Docking 99% of the time.
2009 F150 - 5.4 Litre - 6 speed - Tow Package