Unfortunately, there's no way to have a custom load function or anything more than what you see in the load tab. However, you can put this at the top of your custom trigger function and it won't activate for non-tanks:
Code:
if (select(6,GetSpecializationInfo(GetSpecializaion())))~="TANK" then return end
If you don't have a custom trigger function, make another trigger and put this code in the custom trigger function by itself, then set the multiple trigger condition that popped up at the top to all triggers (I believe it defaults to any). I'm not at my computer so I can't give screenshots or verify exact wording of those options of you are unfamiliar with them.