Long term consequences of trailer towing?

We are considering a tow hitch for our 2011 Prius II to allow my wife to tow the small utility trailer she uses as manager of our local community garden. The trailer she uses is a 3.5' by 5' wire mesh trailer that weighs 250 pounds, and our Prius has about 150k miles on it.

We have read through other postings here and are now familiar the general constraints of a Prius towing a trailer. We have an exceptional situation, however, which is that every 7 - 10 days, June through mid-September, she uses the trailer to move a water-filled 1000-liter tank from our house down to the garden, where the water is then transferred to another holding tank to supply the garden.

She currently uses our 2004 Pontiac Vibe (disguised Matrix) for this task but that car has a limited life span by now. (The Vibe is a front wheel drive auto transmission, 1.8 L engine.) As a result, we are assessing options, including putting the tow hitch on her 2011 Prius. Obviously, with a combined weight of around 2500 pounds, it's well over the recognized (by owners not Toyota) Prius towing capacity. This water hauling task, however, occurs only 3-4 times a month, and the distance is just under two miles, all of it on paved roads except for the last half mile, a maintained dirt road. The route is basically flat but with an initial quarter mile paved downhill section.

We believe that Prius can handle the task but we're concerned about whether using it for this purpose would have any long terms consequences. Does anyone have direct experience with this type of load, or general comments or suggestions?

Thanks much for whatever thoughts you might have…