I would strongly recommend you use a good solvent, clean them out then buy some type of liner or a sealant you swish around in the tank. The sealer would need to be compatible with drinking water.
Gas tank prep before welding? - ADVrider
Then after all of this you would have tanks suitable for shower water and washing but I would not drink or use the hot water as potable as the compounds in propane are much like gasoline in that they are never really gone.
For a shower or rinse water the qty of these materials will be minimal enough that your probably won't care or notice (yes I have taken a shower in water that had a slight gasoline smell once before, my skin didn't dry out and nothing really happened)
The key here would be to bring the water to be tested (yes you can do that, sometimes your local government offices will do it "moderately" cheaply because of subsidies and worries about water quality. key would be you would have to bring the sample to them not them to you. Then if it passes drink away, otherwise showers and rinse water is about it.
Cheers
Ryan