Sealand has a vacuum tester for this problem - you can order it form them.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Dometic has developed a simple tool to assist in identifying the location of vacuum leaks. The vacuum tester consists of a vacuum gauge and a cone-shaped plug. Inserting the plug in the inlet of the vacuum tank or generator isolates the toilet from the system. In this way a troublesome leak can easily be located in either system.
Order only the Vacuum Tester (Part Number 530002) or as part of the complete system maintenance
kit (Part Number 310228). </div></BLOCKQUOTE>
See it at Sealand vacuum toilet manual
Don't know if it would do any good to contact Sealand Customer Service but the contact info is here:
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL