The engine compartment (the DEF reservoir is usually in the engine compartment on later Sprinters) is certainly a lot cooler
ambient temperature on a parked/stored cold soaked Sprinter than it is on a Sprinter that is up to normal operating temp.
Perhaps the DEF degradation/crystallization is significantly less on a parked/stored Sprinter when compared to one that
that is operating a full operating temperatures.
It would seem that Cheyenne's "keep the components "wet" by filling the DEF reservoir BEFORE storing the Sprinter
is probably the best overall option.
If you are going to store it for > than 2 years (the normal shelf life of DEF fluid per AUS 32/ISO 22241-1 and -2 DEF standards) it could be
a good idea to pump out the expired DEF and the refill it with fresh DEF before putting the vehicle back into service.
pfflyer's suggestion to purchase a DEF refractometer and test the purity of your DEF fluid is an excellent idea.
Roger
P.S. Here's a little video on using a DEF Refractometer to test your DEF concentration:
https://www.youtube.com/watch?v=cNvT1esgc40