I think most people now create a second APN that avoids the LTE radio/network. They have to manually switch between the default and the custom APN. There are a few threads in the General section on how to do this. Of course, you want a toggle to do the switching for you. But you will need that second APN if I am not mistaken.
This may not meet your needs, but I use 2X Battery. It's main function is to turn off the data radio(s) when the phone is in the sleep mode. You can configure it to stay off for a short period of time when after coming out of sleep mode. You still receive calls, text messages, and maybe mail. I'm not sure about the last. With the paid version (2.99), you can have it go into airplane mode for a specified time range at night. If you have a need to check mail, Facebook, etc. while in sleep mode, this won't work for you.
But with 2X Battery, I am getting 24-36 hours from the standard battery, I suspect a heavy user might get 18+ hours if they can aford to have the radio off while the phone is asleep.