I would get them tested for Nidovirus. It presents symptoms extremely similar to a "severe" RI. Classic symptoms present as an RI with excess mucus in the mouth. You can do the tests yourself here for cheap. Scattershot Exotics has a video on their Facebook page on how to swab and test for it.
Edit: I just realized you are not in the United States. I will leave the info above for others information, but either way, I would find a way to get them Nidovirus tested. What you describe sounds extremely similar.