At first glance it looks like you are being offered a $70,000 motorhome for $40,000. The first step is to make sure it is not damaged or abused. An inspection is therefore critical. I would get an RV inspector to look at the house side of things, then take the unit into a Cummins dealer to check out the drive train. A cylinder cutout test, maybe dyno testing, injector and fuel pump testing, cooling system testing to verify condition of radiator, Allison transmission, and engine is necessary. This should cost a total of about $800 and is cheap insurance at twice that cost.
If it passes muster find a way to come up with the extra $10,000 and go ahead with your beautiful new coach!