Eglin Aero Club
The Eglin Aero Club has a C-310 and a DE on site that gives ATPs. I did mine there. It cost me about the same as the ATP flight school's advertised price, and I had no C-310 time and very little light twin time when I started. The DE's fee was $500. It took me four prep flights in the C-310 plus the checkride. The checkride wasn't cake by any means, but it was fair and there were no surprises. The DE is a good guy and a military advocate.
Also, I read on Sheppard Air's website that active duty can take FAA writtens at Hurlburt for free, but I have not tried that.