If you are just replacing the inline filter drier, the driers they sell at Frigoboat are precharged w/the self sealing couplers.
If everything goes right when you take the couplers apart they should seal, then insert new one, you should be good to go.
Would start up the unit and look at your frost line on your evaporator. If fully frosting over, you're good. If you need to add refrigerant, add very small amounts and watch the frost line until you are happy. Too much refrigerant will cause the line to freeze outside the box and too high of an amp draw from the
compressor. I run a watt meter to
monitor the amp draw/ah used in conjunction w/the frost line on the evaporator to let me know how well my system is functioning.