I inherited a sewer vent leak (for several years) from PO, water collected on top of black water tank, I cut the floor out to get to it and change out the gate valves seals which were leaking.
It was a joke trying to get to the valves thru the inspection doors below.
After I opened up the floor I found water puddled on top of the tank in soaking wet insulation.
Now it was almost 5 weeks since I fixed the leak but the water was still there, never dried out.
I redid both valves, cleaned both sets of wires connections going to black and grey tanks, then re-routed them a better way.
Next I formed the foil insulation Promax R-14, it was great to work with, a pair of scissors, some duct tape and it can't get soaking wet ever again.
Last cleaned and painted with POR.
Closed up floor and fibreglassed over, see this thread