The same tank was used on 1972 through 1977 according to the Inland RV web site: Inland RV Center Inc.
You cannot remove it from above as there's a framing cross member that spans the center of the tank.
To access it, there's a 1 1/8" thick plywood bottom that slides out toward the front under the trailer. To get the plywood out, you need to remove the Z shaped angle iron that holds the front of the plywood in place. Then, you simply slide the plywood out toward the front of the trailer, and catch the water tank as it falls.
Ok, that is very over-simplified. There are several gotchas in doing this:
1. You probably have to remove the front stabilizer jacks to gain clearance to slide the plywood out.
2. Everything will be rusted in place. Removing the bolts on the front angle iron may be tough.
3. The plywood may have been caulked to seal it in place. If that's the case, you may need to cut the caulking carefully with a putty knife or utility knife.
4. Sliding the plywood out may be a bear. Even if there is no caulk. It has sit inside the angle iron channels for 40 years. Some folks drill a hole in the front of the plywood to attach a rope or come-along to.
5. The plywood supports the water tank. Once you get the plywood about 1/2 way out, you should have clearance to disconnect the water lines and sensors on the side of it. This would be a good time to support the tank as well so it doesn't fall on you while removing the plywood the rest of the way.
6. Lastly, the piece of plywood is heavy. Extra hands to get it out the rest of the way and to re-install it will make it easier.
The tank itself is not heavy, just kinda big. And expensive.
But, having said all that, it's a doable project as long as you're ok with crawling under the trailer.
Look at our Little Girl Refurb thread post #266 for a fairly good picture of the fresh water tank plywood under the trailer. That'll give an idea of what to look for under yours.