Your average aircon guy may not have a clue, DC refrigeration is very much a specialised niche where experience counts for a lot.
What you are describing is routine in the boating world, especially liveaboards and full time cruisers.
There are three usual paths.
1 just buy portables OTS, from top notch Engel, maybe ARB, to Indel-B OEMs, to whatever you find cheap. Two identical units that can switch between fridge and freezer usage gives redundancy and flexibility. Seems a pricey option, but easy and at least in theory you can ship / bring a broken unit to get serviced rather than having to pay a tech to come to you.
2. Build a custom box into your space, just foam and fiberglass, often with a freezer space and "spillover" design for fridge usage.
2A Then just install one of the many kits used, including compressor, condensers or eutectic plates, and electronic controls.
2B Use a custom shop to design and build your kit, huge advantage here is on call support both incredibly helpful during the design and install stages, and over the life of the unit, which may be decades.
Technautics Cool Blue is the best, genuinely stellar tech support from Rich Boren, aka Cruise RO with Charlie Hammel 2448 Carroll Lane, Escondido, CA* 92027
Rich@cruiserowater.com
Rich's cell 619-609-3432
Skype Username:*CruiseROWater
Of course not cheap. Your path may end up working fine, just pointing out alternatives, cruisers are usually cheapskates, but they swear this critical investment is worthwhile.