As soon as you stop place a large box fan under the RV pointing it at the by now hugely hot transmission and engine. You have several hundred pounds of hot metal and coolant that needs to be cooled off as fast as you can manage it. The engine alone weights about 1150 pounds. That is a big chunk of iron to cool down.
The insulation is a good plan as well but it may become saturated with heat.
I'm in a class C and in extreme heat I'll open the hood as soon as I stop. It does make a difference.
In my old diesel class C the dog house got so hot it would burn my right knee--so I added a foam pad that had a foil lining on one side.
Try running just the rear air conditioner while traveling so that it runs more or less constantly.
