GenSet

Typically (but not universally) the transfer switch is set up to give the generator priority over the shore power input when both are present, usually after a brief time delay for the generator to get up to speed and settle down. It should be fairly obvious if the generator is supplying power to the coach or not if you turn on a high-draw device like the air conditioner; you should hear the generator labor a bit more.

It's not necessary to disconnect the shore power, as the transfer switch is designed to make it physically impossible to connect both together simultaneously. It is a very good idea to avoid switching under load, so turning off all the appliances etc. is recommended. Some people as a matter of principle or habit will always unplug before starting the generator, and that's of course not a problem either.

I've heard that you can run it while plugged in. I've also heard horror stories of people doing and frying things. Not convinced either way. Personally I wouldn't do it, but I imagine it's safe. To me, it wouldn't be worth the risk of not taking 30 seconds to go out and flip the breaker on the post.

I agree, it's not necessary to unplug from shore power. The ATS will take care of it.

BUT, I always unplug from shore power before I start the generator.
I don't like to tempt Mr Murphy. That's just me and my idiosyncrasies. ;)

I would either unplug or turn off the circuit breaker at the shore pedistel
It's simply not worth the risk
There is always some one who says that is what it was designed for
Switching with power on,
The answer is no, that's not what it is for
In an RV it saves Changing from generator outlet to shore outlet when setting up camp, and it saves going out in bad weather and Changing back If there is a power failure
In home or commercial use it switches to backup generator AFTER utility power fails, Not while utility power is on and in use

The Works wrote:
We are new to motor homing, but have had several 5th wheels. We have a 2013 Winnebago Itasca Meridian coach with a onboard Onan genset. We full time and I need to start the gen monthly, do I need to disconnect the shore power prior to starting the gen? There is a surge protector/transfer switch where the shore power cable does into. Does this transfer switch auto switch to the gen when the gen is running or is there something I need to do? Thanks for your help/suggestions.

When changing power sources (Gen/Shore) Shut down the active source (You do not need to disconnect but turn the breaker off) for 3 minutes before activating the other source.. Same when switching from GEN to Shore.

Optional.. Turn off MAIN breakers inside RV for 3 minutes, you may start generator during these 3 minutes.

Protects compressors and transfer switch.

Gotta re-state what has already been stated....If you have an automatic transfer switch (ATS) it's "SUPPOSED" to keep any conflict between shore power and generator power de-conflicted....but there's always that chance. It doesn't take that much effort to turn everything off inside the coach, flip off the shore-breaker or unplug the shore power before starting the genset. Just a protective measure to make sure there's not problem. My $.02 worth and it's probably worth less than you paid for it! ;-)

When transfer switch will move from shore power to generator with no blink, going back might be a different story.
When shutting down Generator engine will hesitate for a second or 3, what can make transfer switch flapping, or give low voltage before it switch.
Meaning make sure all sensitive appliances and especially AC are off for the moment.
Who told you the need for monthly run?
I just started my Generac first time after at least 4 years. All it took was pushing fuel prime button for 4 seconds and it fire right up.
But I run carburetor dry and drain the tank before.
My portable Honda generator is known to sit for 2 years between uses and never hesitated to start on old fuel. But again, I run carburetor dry before putting it off.

Kayteg1, I've been told by Prevost and Liberty Coach to run the generator monthly under load. It keeps the 'brushes' clean. Also, it keeps fresh fuel in the system.

So, it sure won't hurt it to run and it will also let you know there is a problem before you start a trip.

I was at a rally last October and the fella next to me had 7,400 hours on his genny (diesel) and it was purring like a kitten. He ran it the majority of our 3 day seminar as we did.

MM.

The 12 years old Generac in my camper has 82 hr and purrs like a kitten ;)
Equipment manufacturers will tell you lot of thing that are advised, but unless you use them in extreme conditions -unnecessary.
I have lot of equipment and toys and doing what manufactures advise would make it full time job.
So I do what farmers do putting their seasonal machines for winter sleep.
"coffee can over exhaust and walk away"