Flies and other insects would eat the flesh pretty quickly. No muscles, no movement, right? I think the zombies would stop walking within two days.

If they’re evil magic zombies, you would still have a walking skeleton problem though. I suggest you bring a cleric with spells that inflict radiant damage. Bludgeoning damage (maces, hammers etc.) work pretty well too. Paladins and clerics and also use the turn undead ability, which will come in handy.