Shower Leak

[H2Odogs]

I believe I have a leaking shower drain. After showering I get water flowing on the lower RV floor from a wall that is under/beside the shower wall. There is no water to be seen on the upper floor the shower sits on.

I have read a multitude of other peoples similar issues on this forum but can't try their solutions since the "notched" nut holding the last pipe to the shower drain is so tight I cannot loosen it. I tried to gently tap one of the notches with a hammer and screwdriver and the notch quickly was deformed (lock ring didn't move at all).

My shower pan has no access panel but I do have space in the pass through area under the shower to access the pipes. I'm sure flexing is an issue as FR only used thin styrofoam for support under the shower pan and I don't have access to put more support in there. I'd hate to have to disassemble the shower to fix this issue. For a short term fix I have caulked around the drain but that I'm sure will fail with the flex that I have.

My question is is there a tool used to tighten/loosen that nut and where could I get one? I'm hoping that glue was not used to keep the nut tight but I'm leaning in that direction.

Thanks in advance for any help.