Here is my take FWIW
1)Carefully measure everything in the vehicle. Even make up a cardboard box the size of the fridges ( and slides) you shortlist.
2)Look at the insulation thickness if you intend to use it up north or in summer a lot.
3)Look at the average power consumption
4)Check service availability. Any Engel, Waeco or Danfoss based fridge will be ok. You should double check brands with other compressors. Especially the $600 specials.
5)Get the next size up from what you decide - and run that through the above checklist. FWIW I think 60l seems like it is a good size generally, especially if you drink beer or white wine. 50l is probably enough if you don't.
This is a personal opinion, and I know I will cop a lot of flack. But here goes. Avoid an engel if you want to use it in hot areas, have a HF radio, listen to AM radio at night, or sleep near the fridge. They are reliable as hell, but their insulation isn't great, and the food temperature fluctuates by quite a few degrees in warm temperatures. Also, they are noisier than Danfoss fridges. Electrical interference noise, and sound type noise.
I'm a big fan of National Luna. It ticks every box except two. Price, and they are not widely sold. Service is not a problem though.