The 75amp PowerMax does not need a 20amp circuit because it has PowerFactor correction while a PD does not.
Yes the Magnetek is hard wired on a 15amp circuit, probably sharing its circuit breaker with another circuit such as the receptacles.
Based on their specs, the PowerMax and PD are about equal in caring for batteries on a long term float but you should still check the battery water once in a while.
I don't know which Onans are which for that clean power issue that affects PDs.
If you do go up to 75amps from 55 from the same converter location as now, you might have to change to fatter wires from the battery to the Dc fuse panel battery lugs and change to a higher amp battery fuse too, so the wire can take the higher amps
Randy can advise on any problems installing either model you choose if you get him on the phone.