If your chassis is a Ford the best solution is to stop it from getting so hot in the first place. It was explained to me that the reason the Ford engine area gets so hot is that the air flow up and around the engine when driving is very limited. The solution I was told was to have a air dam fabricated that would mount directly behind and extending below the front bumper and this would direct air up and around the engine flushing extreme amounts of heat out. This is basically the same thing that you are doing when you stop open the hood and set up a box fan except it is happening as you are driving.
This is what I had done and the difference is AMAZING. Gladys no longer has to have the Good Sam's directory on the passenger floor to keep from getting the hot foot. I no longer have to have to a towel between my leg and the doghouse to keep from being burned. The roof air alone is enough to keep us comfortable when driving. And we no longer have to use a heavy moving blanket between the cab and the house after stopping.