As far as the temperatures go, I think the only way to prevent the extremes is to have some kind of temperature regulation to prevent one or both extremes. Nothing seems all that promising here:

HOT WEATHER
1. A heatsink or fan could whisk away heat while the system is running. However, it would, at the most, lower the temperature to ambient. At night, when the unit would be used most often, heat is not (as much of) a concern. However, the fan could possibly help to evaporate condensation (?)
2. You could have design a water cooling system, possibly using the cold water pipe going into the hot tub. I suspect this would only be moderately difficult to implement, but there are a number of complications. In hot weather, it may cause condensation inside. In cold weather, the thing would need to turn off and there is the danger of freezing pipes, among other things. Perhaps most importantly, it would be relatively expensive and possibly difficult to make look decent.

COLD WEATHER
1. You could put the unit under the cover to the hot tub to prevent it from getting cold, but there is a lot of condensation under there and overheating is a definate issue year round.
2. You could design a water heating system. The water is already heated in the hot tub, so it would be possible to try to pump that over. It would be difficult to figure out how to get the water in/out of the hot tub without looking tacky. There is an issue of all the chemicals, etc. in the water. It would also be relatively expensive.

All of these solutions seem difficult to implement. Fortunately, we have pretty mild weather around here. Assuming that a Rio Receiver could survive a couple years of this abuse, it might make sense to simply cross one's fingers and replace it when necessary.

Are the actual temperature tolerances for this unit posted anywhere?

-Biscuits