Any trailer dealer could change a window for you. Dimensions required and a new complete window including frame could be installed. Just like in a house. This also assumes that a window of the size you would need is available in a slider. Found this site online that might help you http://www.easyrvwindows.com/index.html
__________________
2009 GMC Sierra Crew Cab 4 x 4 Diesel/196 litre Titan tank
2013 Jayco Eagle 28.5 RLS 5th Wheel
Soon to be: 2019 Jayco Eagle HT 27.5RLTS 5th Wheel
2018: 21 days Camping