Your Cummins Onan owner manual should help - it's on page 20 of the more recent editions. It's a rather narrow-focus picture of the plugs in the cylinder head, though, so you still have to figure out how to get access to the cylinder heads. The cylinders/plugs both come out toward the front (the access panel), one to the left and one to the right. You will probably need a universal joint on the ratchet to unscrew the left side plug.
https://www.electricgeneratorsdirect.com/manuals/CO_5500_7000_EVAP_Man.pdf
This "how to" gives some hints:
https://www.justanswer.com/small-engine/6x0b7-access-spark-plugs-onan-marquis-gold-5500.html