:confused:
Relative has a 70's model mobile home with this coleman unit. The 7600B Presidential.
Probably ancient but it works. The question is where does the filter go? I have been buying Universal's and cutting them. I put them inside the door with louvers. I cut strips of metal to hold it in. Problem is furnace seems to overhet with these filter's. Help?

Here are some more possible causes. You are doing the right thing with the filters.
https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313-2.html#post269282