Suggest you look at the connection while not using water -i.e. with the system under pressure. If there is a slow leak or dribble thne, the most likely casue is a cracked thread on the fitting - the place into which the filter screws. Unfortunately, the only answe is a new housing. This is a very poor bit of design given the use to which it will be put.
The most likely casue of such cracks is tightening the filter cartfidge too much. They actually have a small stop built into the thread, but nuless you are very careful it is easy to over-ride this and crack the moulding. Rather than the food oil carver sugguest you apply to this thread, try some of the silicone grease used for pushfit domestic plumbing fittings. it is Water Board approved for drinking water and will stay in the thread much more readily than oil. With this you should not require any tools to tighten the cartfidge - finger force should do. My last new caravan came with the filter so tight that the lugs bent while getting it our and , needless to say, the housing was cracked (replaced under warranty).