Polyethylene (PE) is supposedly the best material for water tanks and canteens in terms of the plastic leaching chemicals into the water. It is a pretty inert material. One thing that will damage PE is UV light
, so sunlight and UV water purification lamps will destroy the structural integrity and the material gets brittle and fails.
If the tank is hanging unshielded under the trailer, stray UV can damage it eventually. (I have seem heating flex duct destroyed by UV in ventilated attics!) It does depend somewhat on how much UV stabilizer is in the plastic. NSF PE pipe is opaque black and has some sun resistance, but it won't last forever in the sun. The translucent PE common in water tanks and PEX tubing is less UV resistant than the black poly pipe and the manuals usually say to keep it out of the sun.
I have no idea if UV stabilizer chemicals are toxic so you don't want them in the tank. That would be something to check into.
UV water purification equipment needs to be isolated away, in a stainless steel enclosure, for example.
Some PE is fairly freeze resistant, so a tank outside could possibly freeze solid as long as it is not completely full of water. Leave 25% of the tank for expansion space and you might be OK.
You can't glue stuff to PE, but it is not hard to weld. Harbor Freight sells a plastic welder tool, and then you need rods made of the same plastic to weld fittings or brackets onto a tank.