The OP was just asking about her ambient temps.
I'm not sure how a RHP can be set up in a tub, but they do help with ambient temps and without destroying the humidity like a CHE.
The snakes I have that are in my tubs and cages and are in my son's room are fine; his room is the warmest in the house!
However, when my boas go into their larger cages, I'll need help with the ambients since they are housed in the living room of my home. I will be using a RHP in each larger cage to take care of this.
Have you looked into reptile cages? I ask because you state that the snake will start off in a tub...
I got all my ball python cages from Rich at Reptile Basics. Here's the cages I have and will last a ball python forever: http://www.reptilebasics.com/2x3-Cag...h-p-16286.html.
He'll put the flex watt into it and also a RHP if you choose. Maybe you want to just get this from the start???