You're up in the northern part of that valley where it's not so bad. If you spent a lot of time down between Madera and Bakersfield, or if you needed -20 degree temps inside the van, the V520 would be the way to go for sure. If you were doing frozen poultry or something, But for 25 degrees and in the areas of that triangle you'll be working in, if it were me I'd do the v300.
Mainly it all has to do with ambient temp, internal temp, truck length, and door openings per hour (and refrigerant used - you'll want the R-404a).
So, figuring 100 degree ambient, and zero degrees internal temp, a 14 foot box is actually too big for frozen. The refer won't keep up. But at an 85 degree ambient it would and will allow for 3 door openings per hour. But that's at zero degrees internal. That's colder than you need.
At an internal temp of just 25 degrees and 100 ambient you'll get 2 door openings per hour and at 85 degrees you'll get 7 door openings.
That should be plenty. It's on the upper operating edge for a 14 foot Sprinter, but the Sprinter has a more narrow box than if you have a business chassis with an added box. The V-520 (not Max) is overkill for anything 12 feet or under, and is just on the bottom edge for a 14 foot box, unless you need it for zero degree temps.
For zero degree temps you need 4 inches of insulation. For 35 degrees you need 3 inches. If it were me I'd insulate to 4 inches and get the V-300 Max. Slightly less cubic feet to cool and the added insulation should allow the V-300 to do it's thing.
Another school of thought, though, is with that setup the V-300 will be working at maximum output for more hours, while the V-520 will be working much less (puts out twice the cooling BTUs). The analogy I'd use is the Espar heater I have. The two main models for trucks and vans are the D2 and the larger D4. I have the D4, which spends most of it's time on Low or Medium while the D2 would spend that same time on Medium and High. That's because the D4 puts out twice the heating BTUs and twice the CFM airflow. So I'd be tempted to spent the extra 2 grand for a unit that won't have to work as hard.
Thermoking has computer analysis that should be able to tell you exactly what you need, but I have a feeling that it'll say either one will work, that the V-300 Max is juuuust enough to do the job, and the V-520 is slightly more than you need. But you're not gonna have that many 100 degree days, or even that many 90 degree days, and as long as you know you won't ever need to get it down to zero degrees in the summer, the V-300 Max is the one I'd go with.