Your description sounds like the sump is leaking and the mechanic is trying to fix those leaks.
Firstly - you need correct sealant - not generic red/white/black/orange/yellow RTV, go to the chevy dealer and buy a tube of GM engine sealant, your mechanic will probably want to kill you when he uses it because its really hard to get out of the tube - It is one of the best engine sealants today - if your chevy dealer is not co-operative then go to a toyota dealer and ask them for a tube of FIPG - same quality but black colour.
total working time of the sealants is 20 minutes meaning that the moment the stuff touches the pan lip the mechanic has a total of 20 minutes to fully install the sump at correct torque which is very low - ask him to use a screwdriver only to tighten the bolts.
the bolts should be cleaned first in a bowl of petrol or solvents
the sump should be checked for straightness.
the sump should be checked for stripped threads in the drain plug and repaired as required.
before applying the sealant - clean the surface of the seal lip with acetone or brake cleaner spray - same with engine - mechanic has to clean about 4 inches into the engine also to prevent oil from dripping from the lip edges of the block as the oil will not let the sealant bond.