Someone else here may provide you with more specific information,
But I would try ICON
They sell holding tanks and may be able to help you out.
The OEM & Other Info Links
post identifies WaVac Plastics
as another place to get tanks. That link takes you eventually to Plastic-Mart.com for RV Holding Tanks
Hope this is of some help.
Mary & Tom (aka Hutch)
2015 Jayco Eagle Premier 361REQS
2014 GMC Sierra 3500HD 4x4 CC D/A
Sunlines: 2005 Solaris T-280SR; 1999 Solaris T-2670; 1998 Saturn T-24A