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There are spacers available to bring the wheels out farther but it is not recommend since this puts alot of strain o n the lug studs and on the hub bearings, this could be potentially dangerous on a RV due to the weight and since its top heavy. You could lose a wheel and or hub while driving.
Swapping a vehicle over to disc brakes from drum is possible. The cost of doing so may not be what your looking for but it is possible on this axle. You will need the discs and calipers and caliper brackets and backing plates and the new hubs and bearings. The cost per side will be about 1100$