We are remodeling a bathroom and have chiseled off all of the tile in the shower area and on all walls. We would like to know if there is a solvent that would take the mortar off the walls. I tried using a belt sander and it is very slow going. Any advice would be appreciated.

The only way to do this is to cut the wall out and replace it with new. Sounds like a lot of work, nah, what your doing is way to much work and time consuming. besides when the wall is removed you now have time to properly fix any plumbing pipes that I am sure need some work, also you can properly insulate behind the outside walls. and install the vapor barrier that should be there as well.
Simply cut the wall away just about one half an inch short of what the finished tile will end up. Doing it in that fashion will make it eaiser for you to hide the cut. While you still need to spackel the joint, You do not have to be perfect as the new tile will cover over it.
If the walls are also part of the project. I would remove all of them as well. It may create more debris, but in the long run you will be much happier with the end project.