If the tire comment is true...
S4.2.2 Tire load limits for multipurpose passenger vehicles, trucks, buses, and trailers.
S4.2.2.1 Except as provided in S4.2.2.2, the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system as specified on the vehicle’s certification label...
-49 CFR § 571.110 - the certification label is on the front street side of the trailer. S4.2.2.2 just requires a 
higher rating if passenger car tires are used instead of ST or LT ones.
Edit: as mentioned below, the tires don't have to add to the GVWR:
S9.2 On RV trailers, the sum of the GAWRs of all axles on the vehicle plus the vehicle manufacturer’s recommended tongue weight must not be less than the GVWR. If tongue weight is specified as a range, the minimum value must be used.
-ibid.
Let Keystone know you'll be reporting their non-compliance with federal regulations to 
the NHTSA, that they will be assuming full liability for any damages caused by their failure, and that if it's not corrected within 2 weeks, you will be returning it for a full refund, based on it not being a street legal vehicle, and unfit for purpose.