Any automobile electrical experts out there?

Here's the skinny.
I have a motorhome and the starter went bad.
I replaced it.
It started right up.
But I noticed that it started then the key was turned to the 'on' position, the position before 'start', which is probably what messed up the first starter.
Replaced the ignition cylinder, no help.
What I discovered was when the motorhome battery was disconnected the motorhome started like normal. No issues at all. As soon as I connect the motorhome battery it starts in the 'on' position and if I turn it to the 'start' position I get the screeching sound you get when the car is already running and you try to start it.
This all just happened out of the blue.
Any ideas why it would be normal when the motorhome battery is not connected but faulty when it is connected?

Sounds like the starter solenoid is bad (again).

Not sure if the solenoid got replaced when you replaced the starter. Some are separate.

Does it have a remote relay battery switch? The relay is used to start the engine if you dtain the engine battery...  (or to start the RV generator is you drain the RV battery..)

Just to clarify, the motorhome battery refers to the living portion of the RV and that there is a separate engine battery, right?

Typically, the battery that powers the living portion is referred to as the RV battery.

So, it sounds like the RV battery is somehow feeding back to your engine electrical system.  Normally these two 12 volt systems are isolated with a battery isolator, Most isolators allow the engine's alternator to charge both the RV battery as well as the engine battery and still keep their loads separate.  They do this with diodes that allow current to flow only in one direction. 

Perhaps your isolator has failed, is wired incorrectly or there isn't one at all. 

A starter has two major components;
1. Solenoid or Relay
2. Motor

The Solenoid is where the electric wires attach.  There are three terminals

1. Hot lead from the engine battery.
2. Hot lead to the motor
3. Switched hot from the ignition key
    the switched hot from the key energizes the windings of the relay, causing the relay's plunger to short out the hot lead from the battery to the hot lead going to the motor, making the motor spin.

Here's a diagram;


You experience the hot from the battery engaging the solenoid in the run position, not the start position, but only when the RV battery is connected. 

Somehow, the hot from the RV battery is wired to the run position of the ignition key and is feeding the starter solenoid's start terminal.

Now, how to find that.  One way is to disconnect the engine battery and measure with a volt meter.  What ever voltages are there, are due to the RV battery and should not be present.  I'd start with the starter motor and see if there's voltage on any of those terminals.  Then I'd move to the key and see if there's voltage on those.  If so, trace back to the RV battery and see what is in the path that is allowing the voltage to get through.  The only thing I can think of is the battery isolator that allows the engine alternator to charge the RV battery when driving. 

If you need a hand, give me a shout.  I work for beer.  Ha! 
Hope this helps.

Does it have a remote relay battery switch? The relay is used to start the engine if you dtain the engine battery...  (or to start the RV generator is you drain the RV battery..)

this.... Battery isolators can also do this. Wiring at the starter is also important. some have other circuits wired with fusible links on the starter + mount. Without more info, ohm meter and walking threw the electrical system. Could be many things (melted wires?) One of the circuits is back feeding power to the start/run part of the system (even an improperly installed stereo could be the cause)
Ski Pro 3 -- Jerry has more info too. (good work!!!)

What a sec!  You have a motorhome?

-Allen

Wiring at the starter is also important. some have other circuits wired with fusible links on the starter + mount.

That's a fact!

Some folks, me included, will look at the battery with it's big hot lead terminal and realize that it's easier to wire stuff up to the other end of that big cable.  Well, 2 cables actually; one to the starter and one from the alternator used to charge the battery. 

Here's something else to consider;
Ground.  Now, normally ground isn't a big issue.  Everything is metal and everything is connected with hardware to everything else, making just about anything made of exposed metal tied to ground.  But your RV battery might not.  Maybe the current flow is through the RV ground back to the engine battery.  This would probably be 24 volts and not 12 volts so don't be surprised if you do disconnect the engine battery and there's no 12 volts to be found coming from the RV battery under there.  It could be that the two are in series and not parallel. 

To check for that, you would need to measure resistance between the RV battery ground and the engine battery ground.  They should be common and measure zero volts. If not, and someone did try to wire up a battery isolator to charge the RV battery off the engine alternator, that could potentially cause some problems.

I'm just shooting ideas out there.  Pure speculation.