I only have one fridge, and have just cranked the temp down to keep stuff frozen when needed (like freezing 6mo of breast milk for a hurricane evac).
There are really only 3 things you otherwise need to keep completely frozen: ice, frozen treats, and meat for longer trips**. If you can do without those things, then you don't really need a freezer. (I normally drink whiskey on the rocks, but was always still more than happy to just drink it chilled in the fridge)
**Putting a frozen hunk of meat from your house into a 35-degree truck fridge will still keep it safe for several days, especially if it is steak and not chicken. So, I'd say this part still only applies for trips longer than maybe 4-5 days.
I probably would second the notion of buying two separate fridges. As stated, simply bring the 2nd one along for the trips where you require the aforementioned freezing capabilities. That also makes more sense to me because in those really special or extended trips, you'd probably need the extra space anyways.