iWatch SE (2nd) charge with USB port

I just bought an iWatch SE (2nd generation). I bought the 20W USB-C charging adapter but I need to see if I can charge from a different source, namely a 12V (?) USB-A female port. To be more specific I have a travel trailer that is powered by solar panels (often) and only functioning with 12 volt electrical devices. It has two female side USB-A ports, but I need to figure out connectors / adapters so I can charge this new watch with out having a 110V power outlet.