Jay is right. I have used a smaller group 24 to power both a 36 lbf trolling motor and a sonar. Their warning about not powering the sonar off the TM battery is the possible electrical interference (noise) you could get if they're both running. But try it and see - may work fine.
Other than that, the 5 ah gel cell he recommended is also a good choice. I have a 12 amp-hr that will run a sonar for days. BUT, you have to be able to charge it with a "smart" charger - one that senses charge level and tapers back when it's charged. Same with your 35 ah TM battery, BTW. Just get some connectors at a Walmart that fit the gel cell and splice them to the sonar power leads.