Four years after being medically retired DFAS says I never submitted a 1351-2?

After four years of being out of the military, I look at my credit report and suddenly see that there is a DFAS balance of about $2900, stemming from when they advanced me funds to move across the country when I got out. DFAS is telling me that I never filled out the 1351–2 and submitted that along with all of the ending documentation. I had no idea, but I know for a fact that I did submit all of the documents. They are requesting I fill out the form again and submit all the documents but, I’m pretty sure I don’t have the 1351–2 or any of my weight tickets anymore

I am unsure how to proceed, they are six months behind with their debt waivers, the minimum payments they want are almost $110 a month, so I’m not quite sure how to proceed. I guess I could try to replicate all of the information for the 1351–2 but, I’m not sure What to do here?