Hi cav123,
Not seeing any feedback on your post, I'll pass along some I have learned from restoring older campers, roof coatings, and slide leaks water damage.
Your camper is new enough that unless there is a defect in the slide roof EPDM membrane or a hole in it from something, your leak is elsewhere than the membrane itself. There are many ways a slide can leak; we need more info about the leaks and how the slide is made to help tell you where to look. If you need help to sort this out, ask away.
Since you had the main roof done with flex armor, that is a good investment if you want to keep your camper a long time. The product you talked about RVMagic. Is it this one?
https://www.rvroofmagic.comI have not used that coating, I'm not saying it is bad, but I see a lot of marketing on the website in favor of their product. Before you go down the coating route, ensure you understand all the needs and prep work before doing the coating, how to repair the roof or roof items after the coating, and, most importantly, do an adhesion test after cleaning before applying the main coating.
I'll pass along my view on roof coatings; if you have an active leak on your slide, the coating may not correct the leak if you do not know the source. And if there is water damage, the damage should be repaired before any brand coating is done. These roof coatings should be considered a preventative longer life measure on a known nonleaking roof. They should be used to extend a sound roof's life, not as the main source to stop a leak by using it.
You talked about slide toppers; there are a lot of personal opinions on using a slide topper or not using one. From my view of doing water damage repairs on slides with no topper, I believe they are good to have in "most" cases. A slide topper sheds a lot of water off the slide roof; the slide ends walls, the slide seals, and the corner transitions from roof to sidewall. It is a good defense tool to help not get a leak. I have found that a topper will not solve everything regarding a slide leak, but it does more good than harm. I can explain why and how I have this belief, but not sure yet if you want or need that.
You asked why you have not seen many roof coatings on a slide roof; I'll give you some thoughts. I have done four camper roof coatings and many total roof replacements restoring older campers.
The slide roof membrane can have a slower deterioration rate than the main camper roof membrane; let's talk EPDM since that is yours. RV-type EPDM has a white shed layer on top and a black layer on the bottom. The shed layer sheds over time when exposed to the sun's UV rays. It slowly thins out the white layer and washes off with the rain. When you see large spots of black showing on the membrane after lots of years in the sun or intense sun, it is time for a re-roof or a coating to build back up the shed layer. The shed layer protects the waterproof black layer from deteriorating.
If the camper is always stored outside, and the slide is closed, the slide does not get sun damage. The slide membrane does not age like the main roof, as the sun is not beating down on the slide roof being closed.
If the camper has a topper on it and the slide is open, the topper protects the membrane from the intense sun.
The point is the slide roof membrane can have a lower deterioration rate than the main roof. It all depends on how the camper is used and stored.
The four campers I coated the roofs on were all slide-type campers. But, only two of those campers had the slide coated. There were specific reasons why each camper owner did or did not get a slide roof coating. It all depends on how the camper is used, how long the owner will keep the camper, the condition the slide roof is in, and if there is a topper on the slide.
I would suggest this, find the reason why your slide is leaking. Correct the leak source ('s) and repair any damage. Then sort out; should I coat the slide roof or add a topper if you do not have one now? If your membrane is in good condition and you add a topper, the topper may add more total overall value in preventing leaks than coating a slide roof with no topper.
I hope this helps
John
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.