Always have had a problem here in central Illinois with condesation on the
toilet tank. Best guess, the water coming into the tank is in the low 50's
and the condensation can be waful when the windows are open and the humidity
is up.

Short of adding a holding tank in the basement to warm the water to room
tempature is there anything I can do? Then only thing I can think of doing
is to hook up the toilet to the hot water pipe. Any other suggestions?