Removed Genset can't start RV

First, I have NEVER owned, or worked on, a Diesel Pusher.

OK, next, picking up on Kevin's remark... Is it possible that with more than one Battery disconnect Switches/Solenoids, One of them is Powered by Another? I don't mean the Battery Power that it is controlling. I mean the Power that Controls THEM. In other words, say the House Battery isn't powered on, then there's no "Control Voltage" to pull the Chassis Battery into play?

Pure speculation likely based on Too Much Coffee...

Battery disconnect switch? Sometimes when I have electric issues I toggle those and things work again. My friend had a rig similar to yours and I think he had more than one - chassis battery disconnect switch was located in rear engine compartment. Worth a look.

Even if you have a rear start switch, not sure it would start without the key doing something. Worth a try if you have one.

Thanks for the responses. I know this is reaching but perhaps some more clarification will help. When I removed the Genset the only cables I disconnected were for the Genset starter (Red positive) and the Genset short black Ground wire from the Genset housing to the Frame within the compartment. I also removed the Control box harness wiring that goes back up under the Drivers dash. I cant see how the Generator wiring for the control Box could have affected anything in terms of my ability to start the Engine. My Class A has four (4) House batteries and two (2) Chassis batteries. My battery isolator Switch is currently turned on for both meaning I have a connection to the Chassis Batteries and the House Batteries. Funny thing I cant figure out among everything else is that the Generator also has its own Battery in a separate side compartment which I thought was used to start the Genset. However, when I disconnected the leads to that battery and removed the Genset and experienced the starting problem, I checked to see if the Generator positive had any current on it. to my bewilderment..lol..It did, so it wasn't coming from that battery because I had both leads to it completely disconnected. So that meant I still had 12V coming from either my Chassis or House Batteries causing an incomplete loop? So..., in an effort to remedy that I removed the starter from the old genset, reconnected the positive and the frame ground as well. Thinking it would complete the circuit as it was, but that didn't work. I checked all fuses and they are fine. My next move would be to try and switch the Chassis Battery isolator button on and off to see if the relay in the Intellitec battery Control Box got hung up. I was afraid to do that because I normally keep it on while the RV is connected to shore power. I've had problems with it going back on once I turned it off in the past so I didn't want to introduce another problem at this point. Again, the Chassis Batteries and House batteries are both on now as well as I'm connected to House Power. Unfortunately, I'm not the original owner and because of the age of the RV wiring schematics are non existent at this point. The one that I do have is limited and does not show how the Chassis batteries or House Batteries interact with the Genset.

what all did you disconnect ?
when you removed the Onan ?
the batteries ?
did you turn the battery disconnect switch back on ?

the only thing you needed to disconnect was the hot and ground at the generator
NO where else
where are your batteries located ?

did you spark something, blow a fuse ?

My generator starts off the engine batteries. Do you still have 12V on the large wire that you removed from the generator? Does anything happen when you hold down the boost switch?

The only 12V wires you'd have disturbed in removing the generator would be for the starter. You're going to have to follow the hot and ground wires back to the battery(ies) to see what's up. My generator starts with the house batteries - there is no connection to the chassis except for a hot wire to a battery isolator that allows the house batteries to charge with the chassis alternator. I don't see how interrupting that current would affect starting the engine. So yeah, check to make sure you didn't accidentally disconnect a ground. Mine has a fat ground on one of the genny mounts but it does not appear to be related to the chassis battery ground, which runs from the battery right to the frame. The engine has a big ground strap running to the frame.

check your grounds