The Developmental Studies Hybridoma Bank has a monoclonal antibody that recognizes p19 gag from avian retrovirus. Would this antibody work in your system?
Article Production of avian retroviruses and tissue-specific somatic...
The AMV-3C2 mAb distributed by the DSHB is used for that purpose. I will quote the researcher that deposited the mAb: "AMV-3C2 Identifies cells infected with all strains of the avian sarcoma/leukemia virus complex. used in the identification of cells harboring Rous sarcoma or avian leukemia virus derived vectors which retain the N-terminal 50 amino acids of gag, e.g. RCAS and RCAN."
AMV-3C2 works for Immunohistochemistry/Immunofluorescence, western blot, and FFPE.