Welcome to the RV community.
Unfortunately, its not really that simple. I think you should stay with the size recommended by the manufacturer. Tires on an RV carry considerably more than a car or small truck hence the tire load rating is key in the selection of proper tires. Think your intentions on trying to save money might be noble but the tires WILL - not might - eventually fail if overloaded. Think about the liability and damages you could face potentially and the possible impact on your family. I suggest you do some research on the Michelin RV Tire Guide to get an appreciation of issues that come into play with RV tires. You should then be able to determine what tires you need and know why you need them. Good luck,
09 Journey 39Z