The soft goods are duckbill valves and the vacuum seal at the bottom of the bowl
Diaphragms on the pump - or replace the pump
I am the second owner of a 1983 International - owned her since 2001
Did a major mechanical refit in 2006 to diesels and while in there decided to PRE-EMPTIVELY replace all the soft goods in the sanitation system while the engines were out.
So heres the STRAIGHT POOP !
the 23 year old system was working fine at that time.
It's now 12 years since i did that maintenance.
THE SYSTEM, NOW 35 YEARS OLD CONTINUES TO WORK FINE - NEVER FAILED - EVER.
I should note that I keep meaning to replace the pump itself, but just havent gotten around to it
The vacuum pump is still the same pump that was there when I bought the boat 18 years ago - who knows, it could be 35 years old......
Simply put, the idea of replacing a vacuflush system with something "simpler" is a bad choice
Maintaining your existing system back to tip top condition would cost less than a full tank of fuel and a whole less aggravation than any other option
That waste tank is for the TOILET ONLY and holds 40 gallons !
HOLY $HIT BATMAN !
Please allow me to restate that: That waste tank is for the TOILET ONLY and holds 40 gallons !
Surely that 40 gallon capacity is adequate to last for some time and should cover you if you dump the tank when you refuel
IN MY OPINION, (don't get pi$$ed off here) doing any other kind of homegrown "gravity" system, chopping out the existing porcelin head, or any other kind of non-factory wild idea is an IRRATIONAL and foolish way to solve this problem.
I hope you will reconsider your options, and avoid a $hitty situation
The REALITY is you have only one.
Your actual cost to analyze the problem and restore the system may be so simple and inexpensive that it surprises the $hit out of you

RWS