Is ram 3500 SRW 6.4 hemi enough?

Simply not true. I have taken several trips with mine, non-towing and regularly get 18-19 mpg. All the trips included interstate, city, and mountain passes, with usually about 1000 lbs of payload. Although the last one I was carrying 2000 lbs of pellets plus the family and other misc. items. For a large gas engine it does reasonably well.

I agree, for a large gas engine it does reasonably well. The Truck Camper Magazine did a fairly extensive test over several thousand miles and found two things. First, the mpg readout was way optimistic and the anticipated 18 or so mpg in the readout was more like 15-16 hand calculalted not towing/hauling. Second, they drove with a 5000 pound camper at less than 65 mph and got 11 mpg overall. That was with 3.73 axles and with cylinder deactivation working as much or as little as it could with the loaded camper on the truck. I would opt for 4.10 gears so the truck gets it's cake and eats it too. With 4.10 gears the cylinder deactivation works more often as the engine is working a little less hard and then when the truck needs power in the mountains it has the 4.10 gears to do that as well.

If you are basing your mpg on the truck's readout, you might consider recalculating by hand. If it is 18 mpg I would be very surprised. But, maybe you are a very conservative driver. That also helps with mileage.