I am/was in the same boat. My x was order to pay half of the medical and child support on a regular basis. My original order was in GA but, I moved to FL. I contacted CSE for a modification or child support and to enforce back medical payments to me as he was paying his child support like he was supposed to.
With that being said, it took about six months for them (CSE) to "domesticate" or "register" my order in the State of Florida. Once I had received my case number for the State of Florida, I filed the motion for contempt for nonpayment of medical bills myself. In hindsight it was quite simple even though I felt overwhelmed by the legal paperwork at the time.
My ex was found in contempt of court and one of the things that helped my case was that I had gathered all of the bills that I could and had sent them to him certified with return receipt several months and several times before I filed my motion for contempt. I copied the certified return receipt and filed that with the motion for contempt paperwork. I had given him several opportunities to pay prior to our hearing, even gave him the option for partial payments. He was forced to pay some to me on the day of our hearing to avoid jail and then the judge granted me a monthly arrearage amount on top of my child support.
I am not in your shoes but I do know that it is difficult to live and support children off of one income. If I lived in the State where the decree was filed and if I were you, I would start with submitting your letters to your ex with the proof of payment or proof that the bills and college tuition exist (even if you haven't paid them), if you haven't already. Give him x amount of days to make a partial payment and let him know that you will be forced to file a motion for contempt if he does not comply. If he sends you something, it is better than nothing and it is a start. If he doesn't pay, file the motion for contempt just like you said that you would. Have the paperwork served to him in Florida (Courts will tell you how to do it). While you are waiting, file with CSE and they will eventually get you your CS money. It takes FOREVER. Include the amount that you know that he owes you in CS, medical and tuition as arrearages. You will get frustrated, you will loose your patience but, you will eventually get your back support and medical bills. They are the only ones with the abitity to garnish his IRS returns.
If you do happen to do any of the above things that I mentioned, don't call CSE, go in person. I wish you the best of luck.