Agree that the hardest part is taking off the old gasket material and cleaning up the goop they leave behind. On my 2008 3400RL, that is a slide-in bulb gasket. You need to unscrew the corner pieces on the outside corners. That allows you to flex the corner and slide out the new gasket and then slide in the new one.
https://elkrv.com/collections/seals/...ray-thick-wall
"This seal is located on the backside of the exterior fascia, that frames the slide room. It slides into a track on the backside."
You can also get it at trekwoodrv.
https://www.trekwood.com/products/14...ed-TPE-1000-Bx
I ordered from elk because they had better shipping prices.