OK, it seems your actual door maybe has deteriorated internally. And I can see now, your door is made different then the newer rounded corner doors. The evolution of Sunline history continues to unfold one camper at a time. I think around 1996 the entry doors changed to the non wood ones.
We have had many members with older camper, 70's and 80's that had their doors fall apart, literally. They where wood frame internally and the wood would rot out leaving no strength left. Yours appears to be upgraded some from that style door, but still may be suffering from internal failure of some kind.
Here is one place I know of that sells new square corner doors. I have bought from them in the past and have had good experiences on the sale. However they are not the lowest cost sometimes. https://www.pellandent.com/RV-Entry-Door-Replacement
The cost of the new door unit is pricey plus the freight. If you cannot see the exact size you have, call Pellandent and ask them. They may be able to get what you need. And there are other suppliers selling them too. It will be a web search to find them, but they are out there. If you need help on that, ask away I'll hunt some more. I knew they had them as a place to start.
Here is another place, I forgot about them. I have bought from them too and had good luck. Price is still high though. https://www.pplmotorhomes.com/parts/...entrance-doors
Another option is to rebuild the door you have. Take the door off the hinges and take it all apart and repair. You mentioned particle board, if you do rebuild it, consider exterior grade plywood at least. The glue on any of this when it gets wet starts to fall apart and actual particle board is glued sawdust. OSB board is a little better but not if it is not exterior glue Plywood still has glue, but more wood then glue. Sealed up well with caulk in the rebuilding process, it can last a long time.
If you do rebuild it, consider creating a post on what you find and how you fixed it. We have several fixing the older doors, just not one on your vintage that I recall. If you get into the door rebuild, and need help, post away and with pics. We can help on how to take it apart and repair. I would have no issues myself trying to rebuild it but I do not know what access to tools etc. you have.
I looked for the posts with pics on the 70's/80's door but they seem to be hiding right now from the search words I'm using. Not the right ones...
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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