This post is meant for everyone who gave me advice regarding hard running/ backfiring Vortec V6:

Today, returned to jobsite with a highly experienced gas truck tech and continued diagnosis. Both of us working together, we removed cover from mixer and removed diaphragm once again.
Immediately, we heard a suction sound effect being made when diaphragm was being removed. Literally had to peel away diaphragm in order to remove it from top of mixer.
Diaphragm looked fine, reinstalled it.
Removed propane intake hose from bottom of mixer. Again, we clearly heard a suction sound effect being made as the hose was peeled off of elbow fitting.
Installed hose and turned keyswitch. Engine started right away. Idle and high rpm quality is very good. No more backfiring.

Not sure where this extra vacuum came from. Might have something to do with changing the LPG fuel filter.