I had a Nova Kool RFS6500 side by side fridge/freezer that would not keep the fridge side below about 45-50 degrees. I added ventilation with fans and did several other modifications without success so I purchased a brand new Nova Kool unit to replace it. My original doors had inserts that matched the woodwork in the boat so I swapped out the new doors with the old ones when I installed the new unit. Guess what, the new one didn't cool any better.
When I put the new doors (with new seals) back on the new unit, it fixed everything. Cools down to 33 degrees (or below) now. Several changes made small improvements but the magic was in the door seals for me. Expensive lesson learned but now I know.
If you have frost happening quickly, replace the seals.