Just bought a 2017 unity and the prior owner installed a 300 watt lithium battery. I believe I should either change out the inverter and the charge controller for the solar, any suggestions and where is the charge controller located?
No need to change out the inverter & solar charge controller, but you probably should change out the solenoid for a dc to dc charger.
If your inverter does not support Lith batteries you might want to change it out.
Check with your Lith battery manufacturer about changing out these devices.