It may not be. Most of the 2nd tanks used in Dodge trucks are just Ramcharger tanks stuck in where the spare tire would normally be but in '72, there were no Ramchargers since they didn't come out until '74. To make it more interesting, '72 was the first year for the D/W body style, so it's really hard to tell what was used back then. Still, the Ramcharger tank design had to come from somewhere so it's also possible they borrowed the auxiliary tank design from the earlier trucks. I think a '72 should use the original in cab tank and then the 2nd tank could just be a normal side tank though, which may open up a lot more options.
Unless you are a stickler for originality you could just toss the stock 2nd tank and upgrade to a Ramcharger tank for better parts availability if it's behind the bumper or a later side tank from a '80s pickup. If you went with a '80s tank before '88 (when TBI was introduced) you could have a 35 gallon auxiliary Ramcharger tank that's plastic with the optional skidplate for protection. Some of the 70s Ramcharger tanks are smaller and some are steel also but the parts can be hard or impossible to locate for them as well. If you have a side tank, you could use just about any truck tank that fits the wheelbase. Since you have a D200 and not a W200, you don't have to fool with one that has to fit around the divorced transfer case crossmember. If you want to replace the side tank, I would go with an '80s tank there also for better parts availability (for now) and you won't have to deal with the rather stupid design of the early '70s side tanks where the fill tube comes through the driver's side of the bed then goes all the way to the middle of the truck and then turns back in itself to go in the tank on the back side. This makes it near impossible for any of these trucks to take fuel from a modern high flow pump. Just my .02
