Welcome Momfleag!
I may be able to help with the key and Fender skirts, but we need more information from you to help better, such as the model year and model number of your Sunline. That way, we can give you more specific answers than general big-picture answers.
First is the key. On the newer campers, the entry door units were made of metal frames and fiberglass panels as an entire door unit. Sunline used many Fastec door locksets on these premade door units. The older doors were more wood-based frames for the door itself, and I am not sure of the brand locks they used. The Fastec locksets, most of the time, have the initials of FIC (Fastec Industrial Corp) molded in the part of the lockset, which can be seen on the inside of the door lockset. The FIC initials may be in other places, not only where it shows in that picture.
Here is a pic of one of them.
If you do not have a key code, the Fastec locks sometimes have a sticker inside the lock set with the key code or a number stamped on the outside handle key cylinder.
If your lock is a Fastec (also known as a Wesco Fastec), the company is
https://www.wescofastec.com/locks-keys. If you call them with the key code, they can send you new pre-cut keys. While you are talking with them, ask if your lock set is in the recall, they had a recall many years ago, and would send you new lock cylinders with two new keys for the new lock cylinders free if your locks are on the recall.
Some RV dealers sell key blanks for Fastec locks even if they cannot cut their teeth. If you can buy the correct blank to fit your lock, then take your working key and the blanks to an older hardware store or locksmith that can cut keys, and they can cut in the teeth. You may have to find a lock key cutting place with the older machines that trace the teeth rather than the new machines like in Home Depot, etc., which are all automatic and work well with house keys but not on these more unusual camper door keys.
Fender skirts. Again, year and model are needed, as Sunline and fender skirts changed as the years went by. Campers with slides can have longer skirts than non-slide campers, even if the year is the same. The tires are bigger on a slide camper, so they need longer fender skirts.
Here is a place where I have bought new Fender skirts for Sunlines before. They have lots of fender skirts and some Sunline's specific skirts.
ICON Direct out of Canada.
https://www.icondirect.com/
They have increased their inventory by at least 10X since I last bought from them a few years ago. My goodness, it has it. Pages 42 and 43 have the Sunline ones. They do not go by year or model but by brand and measurements, as they are an aftermarket molder. Someone sends them a semi-intact fender, and they make a mold and then do a larger run of them. They do sell directly. I have made a direct sale, and they also sell them at other places; I have bought them from Camping World, and Amazon even, as they sell the ICON brand for some items. Shop for price and availability. But, you must know their part number to shop for, so start at the ICON site to find the PN's.
Page 42
https://www.icondirect.com/rv-exteri...kirts/?page=42
Page 43
https://www.icondirect.com/rv-exteri...kirts/?page=43
I'll add this about ICON Direct: their quality is very good on everything I bought from them, including AC shrouds, Shower roof skylights, and fender skirts.
I'll also add that if you still have all the pieces of a cracked fender skirt or entry door bezel, I have repaired them and then repainted them as part of a camper restoration effort. If this is a route you would like to go, please let me know, and I'll dig up some more on the Fender skirts I have repaired.
I hope this helps. Good luck with your upgrades. Consider posting a few pics on your upgrades. We all like seeing them, and it helps others when they do their camper.
John