Sounds like a BS answer. Having a schematic drawing of your M&E systems does not make them anymore liable. If the system fails because of a bad design, they are just as liable rather you have a drawing or not.
There concern might be that with the schematics, you could go in and modify the factory systems, but if you did that it would be pretty obvious. And once you did that, it would be my assumption you voided your warranty by modifying the stock system. At the very least it would be a fight.
So they may not want to provide the schematics for that reason. But again, "liable" is the wrong word here. Also, I might note that with vehicles, the manufacturers do not make all this information available for free. Normally you have to buy/get a service manual for detailed schematics.
2010 Jayco 26BH
2014 Ram 1500 CC Sport 4x4