Replacing a hot water tank

[jhawker21]

I have a question about repacing a hot water tank in my Father-in-Law's RV. The previous owner left water in it over the winter and it rusted out and is now ruined. We began looking at how to remove the old one and best we can tell, the water line fittings for both hot & cold are compression fittings and we're unsure how to unscrew the metal fitting that connects to the water heater without cutting the water lines since they don't spin or disconnect. Any thoughts on the best way to unscrew the fitting on the water lines from water heater?

Thank you in advance!

Eric