Nope, nothing wrong with romex, and all kinds of things wrong with shielded. Tim's issue was that the inverter itself was radiating and conducting - low end stuff will do that. If you want a quiet inverter look for the CE mark on it. That means it will have had to pass pretty stringent tests for EMI and RFI.
Romex is an excellent, cost-effective solution for carrying large currents (like the DC current between a battery and inverter) as long as it doesn't have to flex, because the conductors are solid and not stranded.
Shielded is excellent for low power applications like instrumentation and radios, but horrible for high current applications, unless you are prepared to spend upwards of $50 a foot for adequate gauge. Plus, it would be a couple inches diameter. You see that kind of stuff on big radio towers and in labs.
I had a dual battery setup in my 40 I designed and built in 1986. It was to run a gigantic stereo. I could never hear my little stereo when I was driving home from band rehearsal and gigs (imagine that!
) so I designed and built a 6 channel, tr-amped 12 speaker system. I used a big marine battery as the aux battery, and built an entirely separate wire harness just for the Aux system. I charged the aux system through an isolator (low forward drop dual rectifier) I got from an RV place. It worked great for decades, no problems ever. The whole idea is you can run loads to your heart's content from the Aux system and not worry about being able to start and drive. Alternatively, if you pull a
and leave your lights on, you can give yourself a jumpstart with just one jumper cable