I know that there are many people who use heaters in their rubbermaid containers. I use a glass tank so I can't give you first hand info. However, I can tell you that if you allow the water temp to get below the 70's then you are risking URI's and other health problems. Also, I believe that it is best for you to have a heater because if your turtle ever becomes ill you will have to raise the water temp in order to boost his/her immune system and sometimes appetite.
I know for sure that there are a few people on austins turtle page that use rubbermaid containers. you should go their and ask some questions about it.
Melissa