To the OP,
2004 was the last model year that had the front window and it has a wood frame with metal siding. The same wood frame used for several decades and can be seen by googling and pre-2005 wood framed Lance Camper brochure. Very clearly shows the front end framing detail with the skins removed. Really helpful!
It's very easy to work with for the removing the front window as you can screw and glue the new filler framing where the window is so easily and it will be very strong. Then add an inside same thickness as OEM panel to cover the newly installed framing area and glue it on and simply staple or use 1" brads to hold it on while the glue dries. I'd also add a larger vinyl covered panel overlapping the existing front inside paneling with about 2" to 4" overlap using plastic panel edging so it looks professional and like it was originally done. All construction materials can be had from Rycenga Lumber where you live as I also live there too and have purchased the items. Might be had at Home Depot also which is right across the street.
Now fill all the new frame cavities with either spun glass insulation or well fitted closed cell foam board. Cover with matching thickness wood like a plywood and expanding foam bead seal all around the edges to seal against water or moisture intrusion as the bunk area walls and ceilings sweat from body heat and breathing moisture release especially when it's cold outside. I would then brush on Thompson's waterproofing (Petroleum based, not water based) and let it totally and thoroughly dry. One coat is all that's needed.
Next comes the exterior patch and you have choices. Either standard white aluminum that matches your TC's skin and PM me and I'll tell you where to get it in West Michigan or I can arrange for you to buy thru my connections. It avoids and long delivery time and the protective wood packing which is required and the shipping costs which are an arm and a leg! Usually has to go by truck! The thin aluminum exterior patch can be made to look professional and somewhat original by using standard white pre-painted "J" channel around the aluminum siding and be sure to caulk very well. The "J" channel is attached with staples like TOT 50 stainless 3/8" min length to the original window opening framing and caulk well under it before being stapled.
Maybe you want a flat white fiberglass panel look like the Filon siding. No problem, as it's done the same way. If spacers are needed for thickness matching under the exterior skin, use either some roofing felt layers glued on or if a lot is needed, use a foil bubble wrap cut to fit under the exterior skin. Both are water/moisture proof and the foil bubble wrap also adds some insulation R-value.
It's really not that big of a project (about 2 days after having all materials already on hand) if you are an average capability handyman. Only simple hand tools are needed and a decent sized table top to work on. Have to keep the working on the TC area dry and a large tarp covering the entire top and cab over area and tied down will do that in rains.
