I did this last summer in my '89. 1) Open window & screen all the way. 2) Look in top & bottom rail for grey plastic track insert. 3) Remove small itty bitty screw at both ends of plastic tracks. One is on the inside. 4) Gently pull the plastic tracks out. Be careful, not forceful. These are no longer available. I started with the top, no weight on this one. This made more room for bottom track to come out. 5) Gently coerce this screens to come inside. They also ride on the plastic tracks. You can now bring aluminum screens to your workbench for easier replacement.
Reinstall in reverse order. I cleaned & put a dry lubricant on the plastic tracks. This made installation easier. Some of my itty bitty screws were rusting. Now is a good time to replace with stainless one. I got mine at the local Tru-Value. Bring one along with screen spline as samples.
"Sometimes I wonder if the world is being run by smart people who are putting us on, or by imbeciles who really mean it." Mark Twain
AIR 22749 WBCCI 2349 NOVA TAC TN-6
1989 345 LE Classic Motorhome