If there's any chance of insulation under the tub getting wet, then fiberglass wall insulation would be quickly ruined.

If the problem is with the outside wall, then insulating UNDER the tub might help, but what about BESIDE the tub? Can you improve the insulation in that outside wall before putting the tub back in place?

Dear Michelle,
I had this problem in the mountains of Colorado. Since I had Hydronic heating in the house, I insulated with 3/4" foam and ran 3/4 copper tubing around the walls behind the tub from the boiler. I think you can do this with forced air heat and electric baseboard with a remote thermostat. It will take some advice from your local tradesmen, but you will love the results. I did and got a lot of hugs from my other half.
Len