My holding tank is plumbed to the thru hull with only a ball valve.
The toilet line is teed at that point with a leg to the tank. When I
want to purge, I close off the air vent lines using installed ball
valves, open the through hull and activate a tire pump plumbed to one
of the vent lines, which evacuates the tank in about a minute or two.

No expensive submerged parts. All other aspect of the system are
typical.

Using the air purge allows me to inspect the function of the joker
valve. If ever it leaks back while purging, I will know it needs
service. Total air pressure used in this sytem is about 1 or 2 pounds.
If the pipes can't take that, they would need servicing soon.

Terry K