jw-
my (albeit vintage) coach had two automatic transfer switches - one took either shore or generator, and the output of that went to another transfer switch that chose between the output of the first switch and the inverter. in april of this year, i removed the 2nd of those (and the old inverter) and replaced with a shiny new inverter with more oomph (magnum magnasine). this allowed me to also remove the two old todd converters (house battery chargers) and about 3 pounds of copper.
if you put a transfer switch downstream of the inverter which would choose between shore/gen and inverter, seems all the bypassing would happen automatically. the transfer switch is nothing more than two honkin' relays and some arc suppressors.
i suspect you can find a used transfer switch somewhere to experiment with before buying a new one (if that was a requirement).
wmm