First and foremost, your MBS should be under warranty or close enough that there is a likelihood of a goodwill repair.
Next is that your post might get better feedback in the Sprinter section as your issue is specific to that chassis.
FWIW, over the years (decades) it has been my experience that sending units and or grounding issues are the causal factor for bad readings. But it could be the gauge as well. YMMV
Happy gremlin chasing.
No PMís (they are not private anyway)
Posting exclusively in the Forester/Sunseeker section for the amusement of fellow Forester/Sunseeker owners.