How are you measuring the temperatures inside the tub? Air temperature inside the tubs will be a lot different from the surface temperature. Also make sure that your tub isn't see-through, otherwise your heatgun might be picking up the temperature of the heat-tape underneath it as opposed to the actual tub surface temperature. What I did was place a paper towel (or better, use whatever substrate you are using) and then measure the surface temperature of the substrate.
With my setup I have my heat-tape ramped to 97 degF but still only get 88-90 degF in the tub right above the heattape. Yep, a lot of heat loss with my setup. I would also suggest ordering a thermometer with a probe so you can cross-check the temperature readings with your heat-gun.
Also what width of heat-tape are you using to what size of tub? If you're worried about it overheating then down-sizing the heat-tape width might be something worth looking into.
Last of all, taking a picture of your setup and thermostat settings might help more than written explanations~