If you are getting clutter when the TM is idle you have an issue other than TM interference. I 've gotten interference from my TM even with separate batteries so they are sensitive to minor interference. But you should be able to dial in a sharp screen when nothing else is running. I have a small skiff I'm setting up and I plan to use on of the small sealed 12 volt batteries for my DF. They dont weIgh much and are compact.
The main battery will crank the outboard(9.9hp) and run the TM.
Mark 1:17 ...I will make you fishers of men