From the point of view of your converter you're just looking at a 105 amp-hour battery bank, so I think a 45a will do it (and will run off a 1000w generator in a pinch).
If memory serves you never want to charge at more than 25% of the total amp-hour capacity, so for 105 AH that's just over 26A, leaving quite a bit of capacity for trailer loads with a 45A converter.
Now, if you're still running all-incandescent lights in the Argosy you may have enough load with all the lights on to make use of the extra headroom of the 60a when charging and running most of the loads inside the trailer simultaneously.
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