Chinook RV

I just went outside a few minutes ago in 85 degree heat, and I smelled propane and heard the hiss coming from the propane tank. I had it filled a few weeks ago so its still quite full.
Is it normal for the tank to release pressure in hot weather from some pressure valve like that?

It is possible if the tank is more then 80% full and in the sun for the pressure relief valve to pop off until the pressure in the tank is lowered. If the tank is in the sun, shade it with some cardboard. If in is just from 85 degree weather the tank is way overfilled.

I hope the hissing will stop soon. Make sure you don't get any fire/spark/flame near it. I assume it is outside? Propane is heavier than air, so it takes a bit longer to dissipate than methane or natural gas.

I usually only go refill when I know I am going to get some of it used shortly. Not a good idea to have one at 80% full for too long, and let the tank heat up.

Well it's almost 10pm now, and about 74 degrees outside. Just came home from the store and I still smell propane and hear a hissing sound, although the hissing isn't as pronounced as earlier. Sounds like a slow constant leak now. I'm going to call my RV place in the morning to have them check it out if it's still doing the same thing tomorrow.
I had it filled before a long trip and actually used it for the furnace for a few hours, but I'm not sure at what % level it is currently. I was concerned that it could explode, but it seems that the relief valve is doing it's job, unless it fails to close and allows all the propane to bleed out!

I was in Palm Springs getting gas when it was 120 degrees. All of a sudden I heard a loud pop and my tank completely emptied itself...

You definitely need to call the propane people and let them know. There are two types of relief valves that I came across for gas cylinders, one that will open until the pressure is acceptable, one that will blow and stay open, and need to be replaced like a fuse. I don't know if propane use the same types. Just make sure you don't get flame near it. You should not start the engine and move the rig until there is no smell. This is a messy situation, and be careful.