Version 1.1.1:
I have a DV miscalculation problem too, but its not about decouplers, it's about surface crossfeed. Strangely I swear I didn't have this problem in 1.1.0. Affects MJ too.
I have minimal mods (KAS, KIS, KER, MJ, Stage Recovery), tried fresh reinstall to be sure.
The problem rears itself especially when building (conventional) planes; KER only reads fuel which has an engine on its end. Works not too bad for rockets, but is very very wrong for planes (no engines on its end).
Pic 1 - Only calculating fuel with engine attached on end
Pic 2 - Fuel on main body not calculated
Pic 3 - Calculation ok with fuel lines
Pic 4 - Just to be sure, only fuel on main body, flies ok, calculation = 0
Pic 5,6,7 - Engine on end - Rocket style; is fine, calculates additional fuel attached on wings