We usually dry camp and our FW tank is also in front so we have to suck it up and pull a bunch of water. 40 gal so 320+ lbs about halfway between the trailer axles and ball so 160 lbs on the ball. If you are worried about it you can fill a few 5 gal collapsable water containers and travel with them in the back to help take weight off the tongue.
To answer your original question as long as you aren't over your axle or tire weight on the rear axle I wouldn't worry about it. Going too light on the hitch is dangerous but the more weight on the tongue the more stable it is.
Also two 30# propane bottles don't weigh 60#, the bottles weigh about 25# ea so more like 110lbs. The tare is on the propane bottle itself.
