I found the patent application for that specific trailer model which was filed by Avco Corp in 1968. It has foam insulation! The holding tank configuration is shown in the patent drawings
Note to moderators, this patent document might be good to have in the forum archive files for the various FGRV models. It will likely be the best available reference document for this particular trailer both for the written descriptions, the names of the persons responsible for the design of the trailer and for the dimensioned drawings.
It is the same company that made the Avco motorhomes, they were also made in Tulsa.