GFCI outlet tripping and grey water pump

I am having an issue with our GFCI outlet mounted inside an underground concrete grey water tank (i.e. below-ground concrete tank that stores gray water from laundry, bathroom sink, and shower). The GFCI outlet is mounted just inside the plastic manhole access portal and is never submerged by water at any point (overflow for grey water goes into the septic system). I have a very small, 120v AC Little Giant submersible pump that sits down in our Grey Water concrete tank underground and is there to pump the grey water for light irrigation (garden hose on plants/trees) purposes.

I'm not sure if the grey water pump is the culprit, or the high humidity inside our underground concrete grey water tank is to blame for constantly tripping the GFCI outlet that i have mounted near the plastic manhole cover.

To be specific, I have the GFCI outlet inside a weatherproof "in-use" case (Red Dot) such that the plug from the 120v pump can be plugged directly into the GFCI outlet and still have the "in-use" weatherproof protective plastic cover down. Anyway, I've already gone through one GFCI outlet to where it would no longer reset anymore after tripping multiple times over several months. I put another brand new GFCI outlet from a different manufacturer in to replace the non-functioning GFCI outlet and it has started tripping as well. Sometimes the grey water pump will run fine for awhile (hours) and not trip the GFCI at all, while other times it will trip right away, or will trip a couple minutes into the pumping. I have to go out to the GFCI and reset it and then it's fine for a bit, or will trip again right away. Because the grey water tank is an underground, concrete tank with a plastic manhole for access, the humidity inside the tank can be pretty high, especially in the summer. Do you think the humidity is to blame for the GFCI tripping, or is the pump to blame? The breaker for this circuit has never tripped at any point, so this is a current leakage issue as per the GFCI tripping.

As alternatives, I looked at hardwiring the pump to the circuit, but I would rather not cut into the pump's power cord from a warranty standpoint. Obviously, a tripping GFCI indicates a current leak somewhere...but I'm not sure if the Little Giant submersible pump is to blame, or the high humidity (condensing) is to blame. Any suggestions?