14 volt tv and the inverter squealing.

I think this is what LY was talking about. It is manually operated for the adjustable voltage mode, using a small screwdriver and a flashlight to see what you are doing. A voltmeter at the terminals to see what the voltage is doing as you turn the pot. Available in various amp sizes. This is the 60 amper, suitable for most RV work.

https://www.boatandrvaccessories.com/products/powermax-pm3-60lk-12-volts-60-amp-power-converter-battery-charger-w-led-light

Para 9 and 10 here explain how to operate it

https://powermaxconverters.com/wp-content/uploads/2018/05/PM3-12V-LK-Manual-2018.pdf