No, the only condition is that all the trucks must have been built at some point during the game. So even if you loose them all, you can still build the KT afterwards.
@OP Thanks for the laugh, claiming you don`t have anything against the unit itself and then complaining how OP it is for the entire post without giving any reasons why it should actually be doctrinal (besides the OP thing obviously).
BTW if look at the cost of the tech buildings plus KT and account for the crippled fuel income of OKW the total cost of getting a KT is 636 fuel. That gives you pretty big window of opportunity for crippling the OKW player and preparing yourself. Just saying.
I said I don't have a problem with the unit itself. I have a problem with the fact that all OKW have access to it, and so you will always see multiple KTs come out in team games, which skews their late-game advantage even further to their favor
I don't have a problem with the Ace, Elefant or Jagdtiger, because they are doctrinal, and you are giving something up to get them.