How do I make the refrigerator colder?

Make sure the condensation drain is routed such that it forms a p-trap. This prevents intrusion of warm air into the fridge. Good for at least several degree decrease.

As suggested above, freeze containers of water overnight and put them in the refrigerator during the heat of the day. Refreeze at night. It really helps. It also helps, of course, if your awning is on the same side as your fridge.

I'm wondering if part of why a muffin fan IN the fridge is that by keeping the air moving, it keeps that fin thermistor from being satisfied a little longer?

Your fridge isn't in a Slide, right? Those must have a fan on the outside coils.

What's your Fridge Make and Model? If it's a Norcold, they shut the cooling off every so many (36?) hours as a poor man's defrost. It isn't active defrost like a home unit, just stops cooling an hour or so.

Is it Frosting on the Fins? If not, and in the Dry West, might not be frosting, and it's a Norcold, turn it OFF for 10-seconds or so every day. That'll keep the cooling unit running full time.

If it IS Frosting, needs to be cleared of that frost. I'm doing that now with a pump sprayer with water. Seems to be the fastest way. but the door's still open those few minutes.

Speaking of door, do you have good contact at the gaskets? ALL the way around? On our Norcold, we have to physically push the corner opposite the latch, every time we shut the fridge or freezer door.

Finally, check installation of your fridge. There should be NO dead space above or to either side. And, nearly no space behind. The idea is for the heat of the outside coils to create a draft that passes only through those coils. Some of us here have put baffles behind the coils (between coils and inside of coach wall) to direct draft through the coils. This is specified in installation directions.

I have two little (80mm square frame, 25mm thick) computer fans blowing upward from the bottom of the coils. If you buy those, the very cheapest have sleeve bearings and won't last as long as ball-bearing ones. I still get a couple seasons out of the cheap ones. Walmart has a four-pack of fans for $7.

You can add a 12v muffin fan in the chimney flue to help move cool air over the cooling fins.
A very common solution

Before you head out buy a case of water bottles and freeze them at home. Then when you load up the fridge and freezer fill er up with these frozen bottles. In fact you can even switch them from fridge to freezer at night. Someone else suggested this and it works really really good. Also when you want a cold bottle of ice water you will have one on hand! I also have a 120 ac small fan on the back of my fridge when it is really hot to keep the air moving. But try the water bottles frozen I think you will really be pleased. I have done this several times know and will continue to. The fridge naturally does a lot better when full of cold items. If your fridge has a lot of air space once you open the door the cold air falls out and it has to try to catch up.

Are you running on gas? The input on gas is around 2,200 Btu's - so that if 20% goes up the flue, then about 1,800 Btu's is left to heat the boiler. While the electric heat element will only put out around 800 - 900 Btu's depending on the model, and the voltage input.

Shade the side of the RV that the refrigerator is on, preferably on the north side of the RV.

Keep the inside of the RV cooler. Don't know what temp it is inside, but it helps to keep it cooler inside, so that the delta T between inside and refrigerator temp is low as possible.

Try a fan on the outside, behind the cover. A small 12 volt computer fan will work well, about 4" diameter, will move around 50 CFM to 100CFM. This will bring more air in to blow across the warm coils.

Camping World used to sell a small inside refrigerator fan, it ran on 1 D battery. It would work about 30 days per D cell. You only need it in the middle of summer.

Also don't try to add warm food to the refrigerator. Try to only add cooler foods to the refrigerator.

Good luck!

Fred.