What is the (WD) tongue weight allowable for your car hitch? You may be within the GCVR, but are you under the listed WD tongue weight?
Most hitches for tow vehicles have two different GCVRs, and two different tongue weights, depending on whether you are using a weight distributing hitch or not. On top of that, a weight distributing hitch head usually weighs much more than one that does distribute. So you need to add that on top of the actual tongue weight, when arriving at the actual loaded tongue weight of your trailer.
I'm not familiar with the Flex, but I'm going to guess that the non-WD maximum tongue weight is 400 lb. In which case it might be 600 pounds with the WD bars engaged?
So even though you say that you are within the GCVR, could you be exceeding the tongue weight and overloading the rear of your vehicle?
Then, on top of that, what is the cargo weight of the tow vehicle? Are you under the maximum tow vehicle cargo weight when taking into account the tongue weight added to the cargo weight in the tow vehicle?
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previous 2019 Jayco 154 bh; current 2019 Jayco 224 bh
2011 Yukon Denali XL, 6.2L