Run the solar controller chassis ground back to the main battery neg. buss. If there is a large sub buss nearer with a big wire between the main buss and the sub buss that should be ok. Do not use the hull in any way to tie it in.
Most electrical items, starters, alternators, sensors, etc can be had as isolated units. Often referred to as floating grounds. It's just that they have to be looked for and specified.