The water does get hot after a couple days in the desert, but the 4" fill plug allows me to dump ice cubes into the tank whenever we pass by a WalMart. I also have a 5-gallon insulated CamTainer hard mounted in the bed that will keep water cold/cool for several days. And a fridge/freezer in the back seat. And a 58-quart Orca cooler in the bed. Gotta stay hydrated.
It will work, until it doesn't, and then it can kill you. The pressure rating you see for various schedules of PVC is based upon water pressure. Water is a liquid and is essentially not compressible. Air, which will fill the pipe as the water level drops, is a gas and is highly compressible. Compressed air in a given volume of interior space represents a much greater amount of stored energy than does an uncompressed liquid filling the same volume at the same pressure. If the PVC ever cracks or suffers a significant impact while pressurized, that energy is released suddenly and can result in a very good simulation of an explosion, with shards of PVC flying off at high velocity. The higher the pressure, the bigger the boom and the faster the shards will be moving. You are probably OK to pressurize a well built small tank at maybe 5psi just to move the water, but anything above that is risky. Remember that the pressure capacity of your tank is only as high as the weakest link, and fittings are generally rated for lower pressure than the pipe itself. Remember also that PVC is subject to degradation from UV and ozone, so if it's on the outside of the vehicle you'll need to paint it to prolong the life of the tank. I sprayed the tank on my Dodge with a white Behr primer that is specifically listed for adhesion to PVC. Plastic is notoriously difficult to get paint to stick to, but this stuff actually works. I can check the label for specific product details, but you can get it at Home Depot, and it works surprisingly well. I did prep the PVC by lightly sanding it and wiping down with acetone before I sprayed it, and with a little acetone added to the primer.
I tried to post an answer to this yesterday, complete with OSHA citations and links, but got a message that it would not be posted until the Mods approve it. Don't know what happened or why, but the short answer is that pressurized PVC is dangerous. Water is not compressible and air is compressible. As the water level in the tank goes down, the amount of compressed air in the tank increases. Air at a given pressure (say 100psi) contains a lot more stored energy than water at the same pressure, because water is not compressible. PVC degrades, gets brittle, etc., and can explode if compromised.
The higher you can mount the tank, the better gravity will work for you. If you want to mount it low, you might need to use a small 12V pump to get the water to where you want it. Another alternative might be a small siphon pump driven by a cordless drill. But then you have to carry the drill and a charger for it, and it won't deliver much flow.
I don't have any Scepter cans to look at, but I think they are pretty thin blow-molded units, and will expand some when pressurized. Get one with a bad seam and it could come apart when you don't expect it. You could use a bicycle pump like the one shown in the Roadshower link, but you'll be constantly pumping. You might look at the small battery-operated pumps that come with airbeds and camping mattresses (like Coleman stuff). Those might max out at a pressure low enough to work safely with the Scepter can, and they don't take up much space. Or get a roof rack. You know you want one.
Look in the irrigation department at Home Depot. Lots of flow restrictors and small nozzles for drip systems. You can get a very usable stream from 1/4" drip line.