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First, where is your VHF antenna located in respect to the location of the sonar?
There are two types of interference, conducted and radiated that can get into your sonar. Most likely it is conducted because as you indicated that the radio and the sonar are getting their respective power from the panel and it’s being feed from the battery some distance away. Most likely those power lines are being modulated when you key your radio, when your radio is transmitting; it draws much more power from the battery.
Solution for the radio with the assumption that the it draws about 8 amp: take a pair of wires size 10 or 12 depending on the results of the Wire Table, making sure that you use the round trip distant, take the red and black wires and twist them together by anchoring one end in a vise and the other end in the chuck of a drill, twist to 3 to 4 twist per inch there about. The reason for the twisting is counteract any interference that is picked up. The reason that you need a pair of wires is that you have to have an independent return line to the battery; what current goes up the red wire will come back on the black wire to the battery. If you only replaced the red wire, there is interference on the return wire that will get into your sonar. I selected the radio because it is the source of the problem. In addition, you could add a toroid near the radio to stop the interference from going down to the battery.
