Doesn't matter a bit. If the transfer switch has gen priority everything will switch over to the genset. (Not likely) If the switch has shore priority (typical), nothing will happen at all.
Actually if you are doing this to exercise the genset, I would disconnect the shorepower with some light loads on in the RV. This will let you know the genset is working correctly and load the gen a bit to allow it to heat up a bit more quickly. (and 20 mins is plenty for testing, heat cycling etc.)
Jim and Katy
w/ Shelby and Sealy (Bernese Mountain Dog and a Nice Mut)
HAM CALLSIGN - AI4SR
EE, IBMer, Race Car Builder, now Retired.