There is a reason why ETI doesn't offer a slider window option for the rear. I believe it's due to the angled rear wall.
I know that slider windows are designed to leak slightly in driving rain (even when closed). The water collects in the lower track and drains to exterior through a series of weep holes. That means the window needs to be installed plumb in order to drain properly. If the slider is installed with a backwards tilt, the tracks can't drain and will instead overflow to the interior.
If you want to replace the original frameless venting window, I'd suggest looking for an acrylic unit.
I'm in agreement with Ron, my 21 E21C new rear dual pane slider window (angled rear wall) hasn't leaked a drop of water either. I believe I'm the first one to do this rear window, dual slider, double pane window replacement with excellent results. (I'll be at the Quartzsite Fiberglass rally Feb 2025).
In October 2023 I replaced my rear dinette, large one-piece window with a custom dual slider, dual pane window made by 'Motion Windows'. This window works really great for us camping as the airflow is fabulous when one or both sliders are open while sitting at the dinette. I decided not to get the awning type windows that didn't open much (3" on latest model design) with little airflow & didn't want the acrylic window either. I explored other Escape trailers and what they had for rear windows at last years Feb. 2024 fiberglass rally in Quartzsite, AZ & chose to go this route with the dual sliders. I'm extremely happy with how this window looks & functions, including boondock camping in 113 F weather, the airflow is fabulous & it's a huge benefit. This rear slider window has been installed now for over 7,000 miles & 90 nights camping all over the Western US & Canada.
p/n is 1800 series window. I used the free template from the Motion Windows website
http://www.motionwindows.com that you can pick your own style window & slider type, corner radius, dual pane, tint option...etc. I ended up removing my original window myself to get the exact window frame opening cut-out size & wall thickness measurements to give Motion Windows so they could custom fit my window size to my window opening on my trailer.
When I removed my original window, I reused my original factory window rubber gasket as it wasn't damaged to get the exact window opening dimensions. Yes, it's a little work to remove but mine came out within 15 minutes, I measured the window opening & filled in order sheet, then I reinstalled the window until my new Motion Window was shipped to me 6 weeks later. I used my trailer multiple times during this period while it was raining with no water leaks.
My new double slider window arrived in 6 weeks. I removed the original window a second time & installed the new Motion Window including their included factory supplied gasket in about 2 hours.
It's important to take your time doing all the above-mentioned steps & do your measurements 3 times to be accurate.
Exact cost to my door includes tax & shipping was $700.38
Prior to ordering my new rear, dual slider window I did do angle degree measurements of the sloping angles of 2 trailers - on the rear window of my 2021 Escape 21C trailer & the side windows of my 2007 Casita Spirit Deluxe and they are exactly the same slope angle readings of 8 degrees, so both the Escape rear & Casita both side windows I measured were 8 degrees angle of slope. Both trailers were leveled prior to my angle measurements. My 2007 Casita side windows never leaked in pouring rain with this angled slope in the 8 years I owned it.