I put in a dual group 27 tray so I could run my TM off 24v instead. I wanted to add battery life now & be ready for a TM upgrade in the fall. I also installed an on-board charger. Well I tried too. That thing will not fit in the compartment mounted to the transom & allow me to actually read the LED lights. So for now it is zip tied to the top of my batteries while I figure out a long term solution. Should I just drill a hole into my rear storage compartment to run the wires from there? I think I can afford to give up the storage space. How bad is it for now to have it just strapped down inside its current compartment?

I used to have a 2013 Pro165. I had three 27 series batteries and a 3 bank dual pro charger in there. The charger was mounted to the brackets at the transom. Trolling motor batteries got a dual tray and cranking battery got a single. It was VERY tight. Im talking maybe 1" to spare but it did fit. I believe your boat is the same bilge configuration as mine. Seeing the lights wasn't really an option. I just kind of had to look to see what color the glow was. Not many options with as tight as it is there. Some people put the charger in the box behind the seats, drill a hole and run the wires to the batteries. I did not want to give up the space.

Can you use a mirror to see the lights, or just snap a photo with your phone?

You my friend are pretty clever. I will see if I can view it with a mirror.