Dave, I think it depends on what model trailer you have. In your case, Casita
17s are (by design) unusually heavy on the tongue. That's good for towing stability, and the reason full tanks have little effect on your rig, but it pretty much limits your tug to a light
truck or one of the few stouter SUVs.
Others are (by design) lighter on the tongue to accommodate a wider range of tugs. But that requires more careful attention to loading. In Ken's case, 150-200 pounds of water weight behind the axle
(double what the original design was meant to carry) is potentially enough to upset the balance of the trailer.
I'm not against carrying water. I am in favor of stable towing. Another recent thread drew my attention to the data in Jon V's spreadsheet (post #297 in the thread "Trailer Weights in the Real World"). I had it calculate the percent tongue weight for every trailer in the database. The results were startling, ranging from a high of 18.2% (a Bigfoot
21) to a low of 3.8% (a Hunter Compact Jr.). That range excludes 5th wheel designs.
What was more interesting was that certain models appeared to be more susceptible to unstable loading, and not all were older designs. Carol reported in the thread that one of the seriously unbalanced trailers experienced sway and rolled on the way home from the rally
where it was weighed, so this is not just theoretical. Her assessment was that water weight was one, but not the only, factor in the accident. That particular model had several light-on-the-tongue units in the database.
The data also back up my claim that the Casita
17 is uniquely tongue-heavy among molded fiberglass designs. So, what works for you may be dangerous for someone else.