Is this the only filter you have from the campground spigot to the refrigerator?
Just an observation but the filter maybe doing it's work filtering out sediments from the water supply. When the filter plugs the flow rate and pressure will fall off.
That is why engine oil filters have a filter by-pass built into them to prevent engine damage do to lack of lubrication due to filter plugging.
You may want to pre-filter the water first with an in-line filter before entering the camper. This may prolong the refrigerator filters life.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67