There were considerable problems last season with on line permits that seemed to coincide with the change in government. I too have seen reports of people being refused permits with 1 tons this season. All i can suggest is printing out the SAT regulations and pointing out page 22 where it states cargo capacity and maybe print out the page off the banjercito site where it also states that (The spanish version (the English version mistakingly states GVW)
All theses documents are available at
https://www.mexicocaravans.com/border.htmlIf you can take your truck empty to a scale, preferably an official one and get it weighed empty which will usually come in at less than 7710 lbs or 3500 kg, it is not a bad idea. Any official piece of paper showing weights can help.