The tried and true method that has been serving static model builders for years is still the best for decal adhesion.

After final paint, before weathering, cover either the general area of the decal or preferably, entire build with a light coat of Future. Apply the decal with a setting solution, if it's an irregular surface, use a decal solvent too, recoat with another light coat of Future, then wash, weather, dull coat or whatever you'd like at that, the decal will essentially be part of the tank at that point.

I'd skip the decal solvent if I were using inkjet decals and go with a couple coats of Future to seal.