The biggest problem you are going to have with replacing an onboard generator with a portable (which is what I think you are planning to do) are:
1. Exhaust (as you stated) your exhaust needs to be piped so that it can not enter the passenger compartment or you could have problems with carbon monoxide.
2. Cooling - an onboard generator is designed to be cooled by internal fans, where most portable generators are designed to be cooled by sir circulation around them, which doesn't work well in an enclosed environment like a generator compartment.
3. Maintenance - doing any kind of maintenance on a portable generator is going to be very hard once you put it in a compartment.
4. Fueling - it is going to be very hard to refuel a portable generator once it is installed in a compartment. You also have to worry about the generator fuel being overheated in a compartment.
I would suggest one of 3 alterntaives:
1. Bite the bullet and buy a new generator designed to run in a compartment.
2. Look for a used compartment generator at an RV salvage yard.
3. Buy a portable generator and build a rack that bolts to the hitch or bumper and have it wired to plug into your RV.