Not to be "that" guy but it is very likely the rear axle is over loaded in this situation. The issue is the amount of weight that is behind the rear axle which moves weight from the front wheels to the rear. If you put a scale under each wheel in this configuration the front wheels could be...
It would be nice but this isn't how GCWR (C=combined) works. What you need to use is the GVWR. The "V" is for vehicle. GVWR is the weight of Vehicle+passengers+cargo and will alway be a lower number than the GCWR (which includes the trailer weight). The GVWR will be almost the same for all...