these questions really need to be answered first:

1. so the air temperature in the tank is 78 degrees but is there a basking spot where the temperature is higher

2. Does the thermometer have a probe?

3. You said that the tank had a UTH, does it have a thermostat?

Temperatures are far more important that humidity, they need to be your 1st priority bar none.

Because tubs have less volume they are easier to heat but in a room as cold as yours you are going to have issues no matter what set up you have.