A few comments about the structure and material.
Using larger steel beams spaced further apart to gain the same strength with additional wall thickness for insulation and fewer beams for fewer thermal bridges vs aluminum. Maybe 1.5" x 1" or 1.5" x 1.5". OR 1.5" square for all the main edges/corners, and 1" on the inside or outside plane for fewer thermal bridges.
If square walls using foam sheet in between the beams. Sheets are easy to cut, are square, and can be purchased locally/cheaply in various thicknesses. Seal the edges with spray foam cans. If you go the 1.5" square main, and 1" square filler panels you can simply use the thinner 1" sheets for in between the 1" square tubes, than cover the full span with a 2nd layer of 1/2" sheet to stop the thermal bridges in all but the corners. Window structure can be the 1" square, so the additional 1/2" sheet will help there a bit too.
Use aluminum sheet riveted to the outside ala' the Airstream method. Check Airpark Supply for the different types and thicknesses. I found them to be the best source when doing my Airstream. Rivet using Olympic blind shavable rivets from the outside, or standard wide flange pop rivets can work too if you want to add additional sealant. They make a Trempco product and Sikka products for sealing up Airstreams. Vintagetrailersupply and airstream supply are good sites to use. They are urethane based, and lasted 45 years on my Airstream without leaks, so they work well enough. I used them when modifying/repairing/re-sheeting the Airstream with good effect. Vintagetrailersupply is a better source, but it comes from VT, so it takes a while. Airstreamsupply is OK, but I have had issues with incorrect items, and their communication is lacking at best...especially on backorder situations. Airpark supply was better for sheet aluminum if you don't already have a source. Even if you do have a source for sheet aluminum, you can use the site to get ideas on thickness. I think I used .028". Since I was polishing, I needed Alclad. You can go cheaper if painting, so don't waste your money there.
On the inside you can use sheet metal, ply, plastic, other. Perhaps put a layer of foam flexible insulation just inside the ply (or whatever you choose) inside walls to help reduce a little bit of the thermal transfer at the corners. The screws will still carry some. You know, the 1/4" near cloth like closed cell that they use in similar fashion to reflectix for windshields, etc.
The nice thing about all this is that it is repairable when away from the garage. Rivets can be drilled out and replaced if sheeting needs to be addressed, screws/rivets on the inside as well. Steel can be welded in the field if you have a battery welder, or if not can be welded in most countries whereas aluminum take a higher skill set (as you well know). Sheet insulation can be routed or hand carved to allow wire passages through the walls if you need to retrofit.
Since you have a relatively small space, and are using an F250 which is likely diesel, go with a diesel air heater. You have a fuel source already, they don't have to be vented due to CO2/Oxygen consumption, and put out dry air. Do some Tern windows or other double paned ones to avoid condensation, and you will really only ventilate to cook/shower/comfort. The space you are heating is relatively small, so the diesel air heater should be more than enough.
When you are more comfortable with composites etc., sell this one, take all the info you learned, and built out one of those. You may find that you don't need to though unless it is a weight issue.
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