Don't connect your Sonar units to the Trolling Motor batteries. You'll have interference issues on the sonar units. I went with a larger cranking battery, group 31, for all my boat electrical needs (pumps, sonar, main motor, lights, aerators, stereo, etc. Always had plenty of power to start the main motor even after a day of spider rigging during a Crappie Tournament. Pumps running, sonar, M360, etc. all on, even the stereo is on. 8 hours running all of it and the main fires right up.
Never had an issue taking the grandkids out swimming off the boat either. Anchored up in a cave cranking the tunes, and swimming all day. Fires right up.
I too don't have room for a dedicated electronics battery, but I do have room for a group 31 and it has performed flawlessly.