I am considering purchasing a 3000 watt emergency generator. At this
size, some generators are 110/220 and others are only 110 volt.

My load will be a 5000 BTU A/C and a refrigerator. In addition, I
would like to put 110 Volts across the water heater for 1/4 power for a
few hours when the load could take it.

Assume that the main breaker is off.

I can backfeed 2 receptacles on the opposite side of the box with 2
extension cords and feed any 120 loads within the breaker limits.

But what about the water heater? How would you suggest that I connect
120 across it?