Short answer to you voltage drop is because essentially you doubled your load by wiring in series. In parallel not connections have the same potential voltage.

If you connect batteries in series you double your potential power (voltage) and is you run them in parallel you increase you amp hours.

Bottom line is you need to connect your heat tape in parallel like others have already mentioned. It is a pain to redo but you will get the desired results.

No issues running six pieces in parallel. All have the same temp. Now once installed in my tax there was slight differences because it is six bins high and ambient room heat plays a roll. Don't be surprised if your bottom bin(s) are lower. My top is a little lower too but no insulation there either.