I don't know what brand of awning you have. Do you know what awning fabric you have? A vinyl fabric or an acrylic, more cloth-like fabric? In 1992, Sunline installed a standard awning rail on the camper, but the awning was an option. If Sunline provided the awning from the factory (the selling dealer may have ordered it to come that way), the brand would be A & E (Dometic bought them sometime along the way so it may be an A & E by Dometic.) We are unsure what brand and fabric awning you have, as a prior owner may have added it and not Sunline.
It also looks like you have the lower bracket for awning arms on your camper. That small camper might have a bag-type awing or a rollup-up awing. I can't tell by your small pic. In the case of the bag awing, the bag has the cord sewn in with the bag and slides in the awning slot on the camper. The roll-up awning has different hardware, no bag, and the fabric winds up on a tube.
But this may help. I am semi-sure the awning rail on the camper is an industry-standard size for that time frame. You can check the size of the awning rail for size and measure the width of the cord slot. Here is the small gutter rail with the awning cord slot on it. This is a roll-up awning, but is has the cord you are asking about.
You measure the inside diameter of the cord slot. In this case, it is 3/8". See if yours matches that.
I'm "assuming" it might match; you confirm. If it is 3/8", this is what awning cord is used to fit in that slot.
I have a few different brands of awning fabric in my shop from project campers I'm working on. All of these awning systems are A & E brand, but the fabric has been replaced on some of them, and it is not the original A & E setup. But we can see the range of awning cords that still work in the 3/8" slot.
This awning, I know, is not an A & E brand; the prior owner had it changed out with an aftermarket brand. You can also see the sewing on the awning fabric with the cord in the fabric.
That solid plastic awning cord measures 0.226". This is almost a 1/32" less than 1/4" plastic cord.
Here is another awning cord, a different Sunline camper, with the original A & E awning fabric. This awning fabric cord measures 0.243". This is just under 1/4". I would call it a nominal 1/4" cord, as it may have shrunk a little over the years, and they just have a manufacturing tolerance.
For the 3/8" awning rail slot, they use a 1/4" awning fabric cord. I'm unsure where you will buy the solid awning cord, but I have a place to start. If you have an awning repair shop nearby that makes new and repairs house awnings to size, they may have this type of solid plastic cord. And they may be able to sew it onto your old fabric if you do not have a sewing machine. You most likely have to remove the fabric from the awning hardware for them to sew it on. Again, do you have the bag parts if yours if it is a bag awning?
Cost. They do sell new awning fabric to go on existing awning hardware. The new fabric comes with both cords already sewn into the awing. Depending on what the awning shop charges to sew on a new cord, a new replacement fabric may not be that much more—something to look into.
I hope this helps.