Your rear axle is smaller width than your front axle. Do you know what the difference is?
Example: If it is 3" narrower, you will need 1.5" wheel spacers (2 x 1.5" = 3"). Due to the weight of our Class B rigs, these spacers should be made from steel. Steel spacers are expensive; but not as expensive as wheels, and it allows you to use the same wheels on all four corners (and spare).
Or you could use rear wheels that have a -38mm offset difference than your front wheels. But, you would then have different wheels front & rear.
The steel spacers are the least expensive, and most convenient option. Look at it this way, the spacers will probably cost less than the fuel for your next trip.