There could be many factors that is effecting it:
1) At altitude it could be cooler at night allowing it to catch up / stay cooler
2) Could have more wind to cool down the back of the unit at altitude
3) Parking in the shade at one place and not at the other
4) Different use pattern (doors open more when down lower)
5) More or less food
It could also be that the unit is slowly dying and it is not able to cool as well now as it could before.
With the fridge in my TCer I was able to see over the years that it was having a harder time cooling till one day I could not get it to cool at all. Replaced the cooling unit and it was good again.
With the class C I have now (a lot newer of a fridge) I can see how this one outperforms the one in the TCer. On the class C if I plug it in the freezer feels cold in about an hour, on the TCer would take 12 hours on the best day. The fridge is still not that much faster in cooling but it does seems to keep up better once cold.