It appears to me the corner edge trim is what your pointing out? Its purpose isnt letting water in or out or sealing. Water should not be at that point, the separation or gap at this point because of the rot I would surmise is part of the result, not the cause.
Certainly the corner piece shown isnt designed to let water out of camper (like window sill you mentioned), simply shouldn't be water on that side. Though you could redesign for that I suppose, not a bad idea at first thouht. Where the bottom of wall opening was a wide enough, sloped edge, so any water getting past upper& side seals was directed out. However its above the bottom seal, which if working would mean water directed on top of seal.
At this point even the water staining from water standing inside you showed doesn't indicate original source of leak, due to you currently have total failure of any sealing. Its been leaking for quite some time.
Need a picture of how this area is suppose to look and how designed to seal. Not much help I know. Looked at the write ups in the TCU and the Lance is a bit different than ours. Ours the bulbs seals are on the slide out flange, the hole in wall peremtier has a 3" aluminum angle, that slide seals snug against, where the Lance top & side seals are on camper wall- not sure on bottom, sealing to filon?