Calss C motorhome - On board ONAN generator

You absolutely can. The transfer switch, if you have one, will not let the two power sources simultaneously be connected to the RV (or to each other) via some sort of physical interlocking mechanism. Usually, it's wired such that the generator has priority over the shore power connection if both are powered.

It is not a good practice to have the transfer switch operate under load, nor is it really nice for the generator to suddenly be subjected to a heavy load when it has just barely started up. AC powered appliances should be turned off when starting the generator for these reasons.

What J-D said.

Me? I'd not make it a regular practice .... Just in case Murphy or the Wee People & Gremlins are about and bored.

My view is wired so there is a genny powered receptical that you plug the shore power cord into. The setup prevents having a problem with a transfer switch. It's either shore or generator....although the genny can run independently.

Yes you can. One of two things will happen:

1. If your Atlantis has an Automatic Transfer Switch, after several seconds, the ATS will switch your electrical load From Pedestal TO Generator.

2. Nothing. If there's no ATS, there's a receptacle LIKE a campground pedestal has, on you RV, probably in the shore tie cable storage area. By starting the generator, you create your own "pedestal" that you plug yourself (the shore tie cable) into. THEN the generator takes on your electrical load.