It looks like the model # is stamped on the black-and-metal tag that has "Onan" as the first line of text. I can't get enough resolution out of the photo to read the stamped number, but you can probably read it when you're looking at the tag in person. Maybe you can get the seller to read the # for you if no one can definitively ID the gen set and you won't be near the motorhome before you go to bring it home.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill