Existing 300 watts should easily run your tv, dvd, satellite and charge small items. I do recommend sine wave as some items such as Apple seem to require sine wave. Power brick tends to run cooler in my experience and reduced chance of tv signal interference.
I prefer separate components for easier replacement when something fails.
You can add a small $50 transfer switch to wire the existing inverter to a branch circuit. Two switches would get you two circuits.
Then just replace your 7400 with a PD9260-14.8 and you are golden.
Really need 4x GC2 connected with 4/0 wire if you jump to a 2000w inverter/charger. And yes they have excellent charging systems.
https://www.donrowe.com/KISAE-TS15A-Automatic-Transfer-Switchbestconverter.com/PD-9260C-148-60-Amp-RV-ConverterCharger
