We have found that growing food in a garden is not difficult, but that it is a two step process; growing and then PRESERVING the food that we do grow. In other words, a lot of times the perfectly good food we do grow, goes to waste during storage. Due to our soil type, terrain and farm history, we can really grow some great root crops, but thought a root cellar would be a good way to preserve them.

I have had this stupid 18 wheeler reefer trailer on my land for awhile now, and was going to scrap iron the thing as it is unsightly from the house, but then thought, 'What if we took and excavator, dug a hole in our side hill (I live on a 6% grade) and then slipped the trailer into the hole. If we then covered the top of it with logs, corduroy fashion so it would hold the weight, dozed the excavated soil over the top of it. That would give us a cheap, 8 x 48 foot root cellar with nice wide locking door that was cold, but not frozen.'

Would that work?

It seems too easy.

I also worry about rodents, but honestly worry about rodents regarding any way we build a root cellar.

I think that sounds brilliant.  Root cellars need cool temperatures (33-40F), air circulation and high humidity.  You also don't want condensation dripping on the food.  I think the challenges you'd have would be creating ventilation, preventing it from rusting out and ensuring the roof doesn't collapse.  I'm pretty sure you'll have the roof figured out.  Vents are often set up with low inlets and high outlets.  Then the rising heat passively creates the ventilation.  To ventilate that large a root cellar may take some large pipes.  Normal sized root cellars often get by with a 4" duct.  You may need a couple 1' ducts...  If you're worried about sucking in excessively cold air you can run the inlet underground for a while to heat up. 

A conex is designed to bear weight on it's corners, and support posts. Not on the roof or sides. Here's a recommendation about how to bury a conex so that it doesn't collapse.
https://survivalblog.com/burying-a-shipping-container-o/

Shouldn't have too much trouble with a reefer rusting out.  "Most" are made from stainless and aluminum, not CORTEN like shipping containers are.  I'd err on the side of caution on making sure the roof was strong enough to hold the weight of the dirt you place on it.  How much dirt over the top do you foresee?  How much of the trailer will be sticking out of the bank.  A 48" trailer takes up a lot of real estate when you start using one for a project like this.  Would also make a good storm shelter if you happen to be in an area that has tornado's.  Good luck, and please share pics !!

It is something that I have considered with my own reefer unit.  I don't see any reason why it would not work, except maybe the weight against the walls, which are pretty flimsy and not engineered to bear any weight.  I don't know how many of these I've seen folded up crushed like beer cans on the side of our icy highways; they have no integrity outside of their intended engineered purpose (a lightweight shell on a flat deck to cover and haul as much weight in goods as possible).  But if you can take the pressure off of the unit as you plan to do with the roof, then you have yourself a great idea.  And I'm not sure how much pressure would be on the walls anyway... that's just where my brain went when I was storming about the same idea with mine.       

At any rate, I have chosen thus far to implement my reefer for use as a storage shed/workshop that is also the back wall of my greenhouse.  If I have even a small amount of heat going in there from my main shop (all of this in not built yet!), then my greenhouse not only has an insulated, reflective (white) north wall, but that wall will have heat behind it, boosting it's thermal inertia many times.  So that's what I'm thinking of doing with my 53' eyesore.  Oh and as far as framing goes, there will be posts on both sides of the reefer, with beams over the roof of it, and these will support the roofing system that goes over the greenhouse, reefer, and shop; so the reefer will bear no weight. 

I'm still considering this idea of yours though, Travis, as it is, I believe worthy of second and third thoughts.