I was able to replace the vent tube. I guess my old bones were able
to fit in smaller spaces than I thought. The thru-hull wasn't
blocked. Things are much improved. I still need to replace the
pump-out tubing and the tubing coming from the head itself. There is
still some mild odor. I also need to find a solution to the low spot
in the vent tube. I'll reroute that tube in some fashion so that I
eliminate the low spot. When I do, I should also be able to remove
most of the bend in that vent line when it enters the lazarette.

I want to thank everyone for their assistance and advice.