I just re-read my owners manual, looking for any warnings on carrying water while traveling. WRT fresh water, there was nothing.
There was a warning about carrying black and gray water though. However, it had nothing to do with tanks not being strong enough to carry the weight. Besides wasting gasoline, most importantly depending on where the black and gray tanks were located, they could cause handling problems. Meaning, for instance, with my Jayco 23RB, the black and gray tanks are behind the axles. I guess if they were full, it could take too much weight off the tongue, and this could cause more sway, for example.
It was completely quiet about tanks being too heavy for their supports.
That said, I will usually carry about 1/3 tank of fresh water (approximately 30 gallons) which is just forward of the axles. I will carry more if I need to.
Normally, I will tow with empty black and gray tanks. However, sometimes I must carry black and gray tanks that maybe 2/3 full (25-28 gallons each). Sometimes it's not possible to dump until I reach home.
Unless there is a clear and specific warning about carrying any water of any kind in the fresh/gray/black tanks, then the common sense says it's fine to carry full tanks of fresh/gray/black water, as long as the GVWC of the trailer is not exceeded.
