Just returned from a trip that went from Denver to Utah and back and did all the I-70 mtn passes with ease. Had our 178 with a full tank of water and yes, that tank behind the axle does introduce sway, but did not find it too troubling. Our truck is a 2015 Chevy Colorado Z71 with tow package. Total Gross Vehicle Capacity is 12K. Truck and 178HRE ~8k, so I am ~66% of total capacity. Slowing down from 65 to 62 mpg had a big impact sway and was important to make easy driving. Speed is significant in the matter.
I also found moving some trailer cargo weight ~100lb, forward helped. The weight of the 36 gal water is ~300lbs, not a lot in the grand scheme so I believe it to be a issue of weight balance. I would recommend practice and playing with tongue weight as I found it affects driving performance.