Lazy Daze Owners' Group

You will read in various places that it's not possible to replace the starter in the generator without dropping the generator and sliding it out from under the RV.   This is a challenge in RVs like ours where the height of the generator is greater than the space left between the ground the the coach when parked on level ground.

Following clues left by other RVers, I was able to replace the starter in my Onan 4000 by dropping the generator 10in or so, sliding the greenish case up, and working from the front and top.   It's a tight fit to get to the rear lower starter bolt, but it can be done. 

Replacing the starter the traditional way (great step by step of the process):
https://www.youtube.com/watch?v=JdUejfzgBRE

Gaining access from the top without complete removal:
https://www.irv2.com/forums/f84/onan-400k-genset-removal-from-rv-and-replace-starter-drive-266840.html#post2786640

Instead of cutting cribbing as shown in the link above, I used a collection of 4x4, 6x6, and 8x8 blocks I already had.   Jack was in the middle (with a 1x6 to distribute weight) and block were fore and aft.

The toughest part was getting the generator back in place - I used a floor jack (with a block of wood) on each side to make small adjustments.   It has to go in fairly square or else the top corners can get stuck on protruding bolts.   Resist the urge to drive the 8 corner bolts unless they are very clearly lined up.   I predict you will snap a bolt if you try without having things in just the right place. 

Getting the exhaust off required some PB blaster and a bit of persuasion.

I used this replacement - not OEM, but it was delivered quickly.  
Amazon.com: DB Electrical 410-21023 Starter Compatible With/Replacement For...

Various bolts are 1/2", 9/16", 11mm, 12mm, and T-40.

Clean the various power and ground connections while you have them apart.

Rich