I don't know what the towing capacity might be in this configuration, but having owned dodge pickups powered with the 318, and also having hauled three horse trailers (not with the 318) I would be concerned about two things. You may find the 318 able to handle the load if you live in the flat lands and are not concerned about accelerator response. If you live in mountainous country I don't think anything short of the 360 would be satisfactory. A 413 or 440 would be much better in this environment. Although I've had pickups with 318's in them I have never heard of a true "truck" version of this engine. I know that "truck" versions of the other engines mentioned above do exist, and that they are identified as 360-3, 413-3 and 440-3. These "truck" engines are designed for heavy hauling and are not the same as the 360-1 or 440-1 found in passenger vehicles. (My 32' 1979 Holiday Rambler - Imperial 5000 on the Dodge M-600 chassis is powered by the 440-3.)
My second concern, and maybe it should be the first, is the tongue weight of the loaded trailer. You didn't mention whether you are hauling Thoroughbreds, Quarter Horses, or Percherons, but a trio of any of these is going to put a pretty good load on the hitch ball. Since the distance between the hitch point and the rear axle is much greater on an RV than it would be on most any other tow vehicle, even a tri-axle trailer will require the use of a good equalizing hitch to keep the trailer/tow vehicle combination at the correct towing height. Be sure the chassis condition of this vintage RV is such that it will take the added strain.
Should you decide to get this or any other vintage RV as a tow vehicle and you need manuals for repair/service, take a look at www.autobooksbishko.com,
especially if the unit is on a Dodge chassis. I obtained a chassis manual for my Holiday Rambler from these folks and have been more than satisfied with it. The first time you need to chase down a wiring circuit it will pay for itself.