I'd say that you will have to go back to the dealer to have them open the door for you. that would be the cheapest method. You could call a locksmith, but that will cost.
doesnt mean much now, but this is the exact reason I always keep the second set of keys in my tow vehicle. My buddy somehow locked both sets of his keys in his trailer too. Not sure how he did that, since one is a deadbolt, and the other can only be done from the outside, but he did.
Dave
2016 F350 Lariat 4x4 FX4 SRW CC SB 6.7 Magnetic Metallic
2017 Forest River Cardinal 3850RL