rear main leaks even with new gasket
Are you asking a question or just venting??
Sorry but nobody has a working crystal ball here, we need way more info as to what engine, miles, what you are needing help with etc etc…
Have you tried using a high mileage engine oil?
These oils have conditioners in them that slow down/stop engine seal leaks.
Proper care was taken when installing the new seal, nothing unusual to install it? Crank surface area the seal rides on was inspected for groove or burr? Crankcase pressure has been evaluated to ensure it is not too high forcing oil past the seals?
Just to be clear - you replaced the rear seal, right? Not just the oil pan gasket?
Did you completely separate transmission from engine before installing the new rear main seal? It appears on V6 3.7L engine the new seal can be installed without messing with the crankshaft bearing caps, but special-purpose tools are required to remove the old seal and install the new one. Did you have access to those tools?
My guess is that Tank57 has no clue what he/she is even talking about. The rear main seal is not a gasket.