Don't use the liquid rubber-type stuff until you have recovered the repaired area with EPDM, either the original stuff you saved or new replacement material (you buy it by the foot or piece). Seal the seams around the patched area with Dicor or similar lap sealant. Then you can coat over it if you feel that is necessary.
here are lots of good coating products, e.g. from Dicor or Alpha (who make EPDM membranes), or companies like Liquid Roof, Kool-Seal RV, Henry's, etc. Take your pick.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC