Storing comsumable dry goods in your car in the summer?
I'm going to treat this question as "how do I keep my home cool when it's a tiny home (often vehicles) and solar gain is my enemy"? Because that would be on-topic here. 

Solar gain is your enemy

And to be clear, it is an order of magnitude worse than any other source of heat, even ambient temperature.  

From the sun's perspective, it casts 100 watts on every square foot - from its perspective - and that includes the top and sides of your van.  This is then reduced by your vehicle's albedo, or its reflectivity.  Black vehicles have low albedo, and a pure white can have as high as 92%.  Also some of the surface heating transfers to the air near the skin of your van,   But with 100 w/sf, there's plenty left to heat up your van.  Windows absorb almost all of it, and cause the sun to heat up things in the interior of your van, where the heat can't leave. 

Get it in the shade

Top of the list is place your vehicle in shade. What am I saying? Top of the list is get out of Florida... Mark Twain once said "the coldest winter I ever spent was a summer in San Francisco" and he's referring to the coast effect.  Santa Cruz, Eureka, Portland, Seattle... Seattle still has to deal with solar gain, but only 3 days a year ;) But I digress....

In shade... I'm saying that there's a huge difference between attaching something to the surface of your vehicle and stood off above your vehicle.  The difference is airflow beneath it that carries the heat away, which is what makes it shade instead of attempted insulation. Solar panels are a great plan, but you need to stand them off significantly.  That will reduce your high speed fuel economy somewhat, because you are also increasing the aerodynamic "wetted area". 

You might also consider deployable shades or solar panels on the sunny side - I assume you can select which side of the street to be on.  Again don't forget the objective is shade and to create easy rising airflow for the hot air the shades collect, so a series of angled surfaces with gaps between them is just fine.  Figure out the sun's angle etc. 

Increase albedo

After your shading options are exhausted, you want to make sure everything is lily white, and have the albedo/reflectivity conversation with your coating supplier.  

Windows have basically 0 albedo, so you really need to address those - the best option is place them in shade.  

Insulate like crazy

Metal is a good conductor of heat, so the car's body will carry heat some distance beyond the area actually in sunlight.  Also, even shaded areas are subject to ambient air temperature.  

Of course you don't have 24" to give up for a deep layer of fiberglass batt, so you need to go for the most efficient insulation you can obtain, and you need as much as you can possibly have.  

Storing temperature-sensitive dry goods

As for storing dry goods, once you've done your best to make your space temperate, your best bet is to put it in some sort of heavily insulated box.  We'd also put as much "thermal mass" in there as possible.  The best thermal mass known is water (coincidentally). So freeze-friendly water-based things, like commercial ice packs, are a good choice for "thermal mass".   

As a bonus, water also has a phase transition (freezing/thawing) at a semi-convenient temperature, and that gives it a huge amount of additional thermal mass if you're willing to cross the zero degree C line, however if you do that, you have to worry about condensation/sweating on your dry goods. 

In a perfect world, your well-insulated box would have a small Freon-cycle machine to chill the box furth-- wait a minute.  Did you say you have a freezer and you just don't run it?  Run it!  You have a solar panel  for Pete's sake, I bet that panel is "dumping" lots of excess energy you can't store.  Use it for this. 

If you don't run your freezer much because you have to run your van's engine to power it, then your solar system needs a lot of work.  Let's have a conversation about that.