Adding weight to your motorhome will decrease mileage. How much depends on several factors. If you add a 4,000 pound toad to a 16,000 motorhome, you just added 25% more weight. If you add the same 4,000 pound toad to a 40,0000 pound coach, you only added 10% weight. Keep in mind that weight is pretty much only a factor going uphill, so percent uphill is a factor. You will also have a larger impact on gas motorhomes than a diesel due to the difference in torque curves. For most situations you are probably talking tops 1/2 MPG difference with or without a toad.
Another consideration is how easily accessible a rental agency is. We've camped in many areas where rentals are a long way off.