First time using Solidworks

Welcome to SW. Welcome to GrabCAD.
Make sure to work through the tutorials built into SW.

Photos taken of monitors are terrible. Actual screenshots are appreciated.

I wouldn't use a 3D Sketch for this, unless the geometry is really weird. A 3D sketch is an added layer of complexity.

Posting your model, or a similar model here will allow people to resolve the issue without needing to reinvent it first.

Finding the volume is easy, once you have a solid (or even a surface) model of the tank made.

I attached an image showing a sequence of converting a 3D sketch into a tank.
3D sketch, then use Boundary surface to add in the walls, Then knit all of those surfaces together to form a solid.
That's either the volume you need, or you'll need to shell the resulting part to give it a wall thickness so it represents the tank.

There are other ways to do this, but I'm guessing you just have a single 3D sketch. So Boundary Surface is good because it allows use of the Selection Manager to choose individual segments of the 3D sketch to form surfaces between.

You have approached the problem of designing and modeling your fuel cell in the worst way you could have, especially if you wish to make a drawing from the finished model to make the fuel cell. As you will make the finished tank from aluminum sheet that will be folded and welded together, using the sheet metal modeling tools to model the parts that the cell will be made from is the only sensible or practical way to go. These parts will then be joined together as an assembly. Accurate drawings can then be produced from the modeled parts and assembly. The software can take the folded parts and flatten them into the flat developed shapes which you can use to cut out the parts from the sheet then fold into the finished part. Any thought of using 3D sketches and creating surfaces for modeling the straight forward fuel cell is basically stupid. The sheet metal modeling tools in solid works are designed for turning a concept like your fuel cell into a physical object that you can make, this is the purpose of these tools.

If you have absolutely no experience or knowledge of using CAD as a tool in the manufacture of things you first need to learn the basics of using solid works. The place to start is to get into the various video tutorials available on line, some are on the solid works web site and there are many available on YouTube, as with any new tool you start by learning how it works and how to use it. If you come from a practical background and know how to go about actually making the fuel cell, once you have learned a bit about how to use the CAD tools to model the cell as you would actually make it you will be able to produce models and drawings that will be better than many that are on GrabCad.

Here is my cardboard model of the fuel cell. As you can see its shaped a little weird to get the best fitment in the car.

The basic CAD workflow I would use if the desired outcome is to end up with an actual fuel cell of known volume that you can place into car is this :

Step 1. Use cad to model the volume of fuel. This lump of fuel can be given real world properties e.g relative density so that the mass for the full fuel load, center of gravity, and much more useful data can be extracted from this model.

Step 2. Once you have a fuel volume that fits in the available space we create sheet metal parts to contain our volume of fuel, making use of our knowledge of the tooling we have so that the pieces we design can be bent with this tooling in the press-brake we have and we keep welded seams to the minimum.

Step 3. We use our CAD tools to produce a set of drawings to manufacture the fuel cell. Because we used the sheet-metal tools in our CAD package producing the flat developed shapes to cut out are created automatically. We then bend the parts into the required pressed parts and weld them together.

The attached images give an indication of this workflow. The actual fuel cell will also have the required fittings, inspection hatches and the like that you will need in a real fuel cell.

Of course for a one off, your cardboard mock up, some straight forward arithmetic to work out the volume, and a good tradesman to build the cell from the mock up and details you provide him with would probably the cheapest way to end up with the fuel cell installed in the car.

Here is what I have played around with so far. Its my first go so I think I can clean the edge up and close the gaps up a little nicer. Its just hard to get the two front pieces to angle correctly and its not possible to put the welded edge on anymore.

183/5000
No need to sketch 3d which are complicated when we have no references plans. On the contrary with just 2 sketches 2d and a smoothing we can easily model this form.

In the end you only have to find the appropriate plan (s) to do a removal of material. (Approximate shapes)