Slider trim/gasket issue

Can anyone tell me what this piece is called?  I keep calling it a slider seal via search engines and get the gasket between the slider and the TT... if you know what to call it, have you ever replaced it?  debating a do it yourself approach vs. taking it in...   Any help would be appreciated! We moved from WA state to upstate NY this winter (February) pulling the trailer behind us.  This appears to be the only issue we have with our subzero, highway odyssey!