We both have the same vintage of Casita. Mine was very chalky and dirty when I got it. I have used both Poly-glo and RedMax Pro (now called Zep wet look)
You can find the later at any big box store like Home Depo or Lowe's.
Poli glow is more expensive https://poliglow-int.com/
I've done 7 different Casitas using both products. More have been with RedMax. The longevity is about the same for both 2-5 years. On the long side is the ones covered or stored inside a building. I know of one guy south of her that has his still looking good after 8 years. I use to keep my covered when not in use with a Calmark cover and it lasted 4 years. Where it will peel or yellow is the top and whatever side sits in the sun. I've done mine 4 times over the past 10 years, it is due for a refresh this year on the top and front.
The key is the advanced prep. Getting it very clean. I us barkeepers friend to get all of the stain and sap off of the surface. What ever is on the surface will remain there if you cover it with either product. Pick a day that is not going to be real windy so you don't have "stuff" blowing and sticking to the surface. I find a sunny day in the 80s works great.
It is very "runny" that is a watery consistency so don't just slop it on or you will have "run" marks. I first experimented on the roof where no one would see my mistakes. You don't have to get up on top, just get on a ladder and experiment as far in as you can reach. Since you have both vertical and horizontal surfaces to practice on.
A lint free small cloth has worked best for me. Something kinda of small not a big sloppy rag. I've tried sponges and it has not worked for me. The Casita will suck up a lot of finish before you will see any changes. It might be 3-4 coats before you notice it starting to look better. Just about as fast as you apply it will dry. Start in an area and work your way around the Casita and when you get back to the beginning, start another layer of finish. Make sure you don't get your rag dirty because that is what your finish will look like. I had to help a friend here in town when he made the mistake of letting the rag get dirty and he had big ugly swirl marks all over his Casita.
The part that takes the longest is the "prep" to get it real clean. I generally put 5-6 coats on my Casita. It looks great and cleans up much easier after each trip. Bugs & dirt don't stick as tight. I usually get everything done in the better part of a day.
Don't put it on your door or any of the plastic accessories like the vent caps or the plastice stove vent.
It is not that hard to remove. I had to experiment a lot to find the best way to remove the finish. I tried all of the prescribed methods from Poli Glow and RedMax/Zep. They gave a mixing ratio of water:stripper to remove the stuff. Both were barely effective and way too much work. I got frustrated and used full strength Zep floor stripper in a spray bottle. It works best with these exact conditions. Have the Casita bone dry have a water hose with a spray nozzle in one hand and the floor stripper in a pump bottle in the other. Spray full strength stripper on the area you want to remove the finish and you will almost immediately see the finish turn yellow and start to slide off in sheets, immediately spray down with a lot of water. Only do a small area at a time. Then move onto the next area. I use this method on a smaller area if I have made a mistake in my application of a new coat of finish.
I tried Marine waxes on mine before I went the RedMax Pro road and had no luck in it looking really good or lasting more than 4-6 months. I even hired a boat detail guy to buff it out with his power equipment. It took him just as long as doing the RedMax and it didn't last much longer than 6 months.
There was a long term heated discussion ArizonaEileen kept going on the other forum praising Poli Glow because they touted having some sore of UV protection. I could not find anything in their ingredients that indicated this to be a fact. I believe she found out as did others who reported on that forum that it was not better than the RedMax Pro (Zep).
There is not going to be any "easy way" to keep our Casitas shinny as they age.