Replacing rubber seals on camper

First take some photos of the seal..noting if there is any "corner cuts or angle cuts"..if there is..take some measurements and do a drawing of their locations.
The "D" seals do not really shrink all that much, what does happen to them is that they loose some rigidity over time and become more compressed and do not reform back to their proper sealing shape..hence dust can get in.
Measure the seal.. width and height..noting that the height would be compressed slightly so allow for that.
Removing is straightforward:
Have a good quality plastic paint scraper..scrape off as much of the seal as possible, there will be some adhesive still bonded to the body.
You could use metal paint scraper..but be careful of the painted metal under the seal you are scrapping off.
Then get some good quality automotive "Thinners"..so with a slightly wet rag..wipe over the seal residue left on the camper.
Initially do this is a small area noting if their is any staining to the metal, discolouring etc., If OK..proceed to eventually strip off the remnants of the D seal along the whole length.
You may need a bit of elbow grease for this..but take your time.
Allow time for the area to dry..run your finger along the area and "feel" with your finger if it is dry, then smell your finger for any residue of the "thinners" .
If there is still that "Thinners" smell..then with a dry rag..wipe the area throughly..and run through the process again.
It needs too be completely removed and the area dry for the ne D seal will not Stick properly.
Attach your replacement self adhesive "D" seal.
It is pretty straight forward, but can be time consuming.
The suppliers...1/. Dunn & Watson. 2/, Ovesco 3/., Industrial Rubber Suppliers 4/ Trailer Parts Direct 5/, Rubber Seals Australia.
They each have their own websites with good info.
Then if you feel not up to this for DIY..head off to any competent Caravan repair place near you and get a quote.

Cheers @peter g28! That detail is super helpful and much appreciated.

Another good supplier to look up is Industrial Rubber Supplies, picked up a few different rubber extrusions from them today
Here

Rubber seals tend to shrink over time, particularly when exposed to UV rays and weather elements. Depending on the age and condition, they may shrink by 5-10%. It’s a good idea to measure the geometry dash length of the old seal as-is, then add a few extra centimeters when ordering the replacement to account for this shrinkage.