Propane. Anyone else have a tank leak with a closed valve?

So I obviously have a leak but I have never run into something like this. I filled both 7gal tanks for an upcoming trip several weeks ago. I ended up hurting my back and the trip was cancelled. I wasn't moving around much for weeks and hadn't gone into the camper until yesterday. I smelled a strong propane smell as I approached the camper. I opened the compartment and turned on the valve on the front tank and it was 1/2 empty. I just put on a gauge from Camping World. Not sure how a tank with a closed valve could leak that much. I pulled the front tank out and checked the rear tank (valve was open) and it was also 1\2 empty. Everything that runs off of propane was off. With the valve open on the rear tank I get possibly having a leak somewhere but not on a 3 year old tank with a closed valve. Anyhoo......I disconnected both hoses and closed both valves and let everything air out. Tomorrow I'll reconnect and use a soapy solution to try and find the leak.