I think your system is similar to (if not the same) as this:
https://www.forestriverelectronics.c...c-s-system.htm
So you should have a plastic tool, like the one shown, that fits around the filter body and allows you to unscrew the body from the fitting at the top. Once the body is loosened you can remove it, and the filter element, replace the element, and reinstall the element and body on the fitting.
A couple of notes before you do this - first, make sure your pump and fresh water connections are off. Then open a faucet to relieve any pressure in the system. Then go ahead and remove the filter body - but have some towels around as it will still spill some water.
I don't know where you'd get one of those handles if you didn't get one with your RV...