I can't say I have completely solved the problem with my 5.0 cu ft, but it is a lot better after a number of modifications. In 90 degree plus temperatures I've been able to keep the refrigerator below 40 degrees and the freezer below 0.
I added a curved baffle at the top of the back side of the cooling section, added insulation everywhere I could, sealed the cooling section from the refrigerator with aluminum tape - in fact, I covered the entire rear of the Refrigerator with aluminum tape to reflect the heat away from the refrigerator. I added an interior fan (see my trailer modifications
page for the fan). I also added a layer of aluminized bubble wrap to the inside of the freezer door.
I also added a fan behind the upper grill that I use when it gets over 95 degrees out. It doesn't help much below that.
All in all, a big improvement over what I had. With the addition of a furnace filter covering the lower outside grill and the additions to the rear section, I no longer have a problem with the gas flame blowing out while driving. While the filter alone helped, the additions helped more.
One additional note - Be careful depending on the refrigerator thermometer linked in Jim's #6 post. I purchased one, had it replaced by the manufacturer, and in both cases, it stops sending (or receiving) on at least one channel. Since it doesn't show any indication that the readout is frozen (pun intended), you can be surprised...