Generac Generator

Sounds as if you have the dreaded Generac starter problem. Mine had 985 hours on it when I got the MH,used,and started right up,for a month or so. Then it started doing what you're describing and then nothing. Took it in to the shop and he said the Bendix was melted to the starter and the starter actually looked pretty new. Found in the P/O records that it only had 20 hours on the gennie since it had been changed,but P/O hadn't put down the name of the shop. We changed the starter 3 times before we found one that lasted more than 10 starts. The last one lasted a year and now has started doing it's slow turn and finally a click. I redid the grounds and it didn't do any good. I did discover that my 4 y/o batteries were toast and so got 4 new 6v and still no good. Battery boost crossover doesn't do any good either nor having the engine running.While trying to recharge the old house batteries,they wouldn't take a charge from either the onboard charger or my 25amp aux charger, I slid under the front and hooked the battery charger to the gennie cables and after 6 hours it went from full 25 amp charging to a 5 amp charge and I tried to start it with the charger still hooked up and a 75amp push from it. Nope... No good. I am going to try disconnecting the battery cables to the gennie and just hitting it with the 75 amp starting boost from the charger. When I don't know as I'm in the middle of a bathroom emergency rebuild. The Dw decided that since I was redoing the totally plugged galvanised drain pipe and had the vanity off the wall,that now would be a good time for new flooring,vanity,toilet and wainscoating on the wall. Kinda over my head,but I'm actually muddling thru it towards a sucessful and neat ending.

Sure does sound like house batteries. You should have a booster switch beside the ignition that will borrow power from the chassis batteries while you start the generator. You have to hold the switch down. It a momentary switch.
You should also check the water level in the batteries. If they are low, that can cause then to not hold a charge. Fill with distiller water only and just cover the plates. Don't overfill.

Since you have a mechanic coming out. I'll wait until he does his thing. Until then it is only guessing.

sounds like (house) batteries are low to me....especially when you consider you were able to start the generator w/ the coach running (most coach engine/alternators charge the house batteries when the coach is engine running)

Id put a voltmeter on the (house) batteries w/ everything turned off & see what the charge level is, my bet is you're gonna show low voltage.

couple of choice if the (house) batteries are low (v) - run the generator (to charge the batteries), connect to shore power so the converter/charger can charge the batteries or drive/run the coach so the engine alternator can charge the (house) batteries.

if the coach has been connected to shore power then i don't know why the battery voltage would be low - unless you've been cranking not he generator starter a bunch or your converter charger is not charging the (house) batteries.

It does sound like low voltage to the battery that the generator is connected to. While the engine is running then the alternator is putting out 13.5 volts (or more) and that will allow it to start quickly. It will be a expensive mechanic call to have them tell you the same thing, if nothing is wrong with the battery. Yet if you have a defective battery, the mechanic might be able to tell.