There are way to many variables to do he math on what your mileage will be with the change of tires. You can do the math on the size of the tires, if the new tire's circumference is 10% larger then you will cover 10" more ground with each spin of the wheel, that is how the speedometer reconfiguration work. Theoretically one would think this would also work for fuel mileage. In reality though the increased weight of the larger tires and increased friction from the larger surface area will cut your mileage, much like how putting weight in the bed or pulling a trailer will do.
Your best bet to get the actually mpgs is to just keep track with the dash display. Or fill your tank then do your normal driving with the trailer for a couple days, then refill your tank. Take the miles driven and devide by the amount of fuel used.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!