Gas caps can go bad. Look at the rubber seal and make sure there are not cracks or other defects. Also, the spring may be failing.
If you don't find any loose hoses or problems in the Evap system it could be the cap. If you only have 10 K on your unit is it may still be under warranty. The Evaporation system is probably part of the emissions control system which have extended warranties (5/50) by Federal law. Check with your chassis brand dealer.
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