I never seen the dull coat make the decal and future look thick. Futre is good at self leveling unless you have a really old bottle. Do you coat the whole plane with future then decal or just where you put the decal? I coat the whole plane and normally brush on the future. Sounds like you are just putting the future where the decal is. If so coat the whole plane with future, allow to dry, apply decals, dry, and then apply another layer of future. Then dullcoat the whole plane when the future is dry.
Now I have seen it happen that the future will pool up in spots. I think this is because my bottle is getting old or I had too much on the bush. If I notice it before it dries I brush it out. If not the second layer of future helps to level it out then after the dull coat the pool is unnoticable.
I have also seen setting solutions make future look cloudy. Normally this goes away once the setting solution dries or dissappears when the second layer of future is applied.
Hope this helps.