J-trim is the thin plastic that protects sheet metal edges on the interior. It gets brittle and breaks away. Replacing it involves removing & replacing some pop rivets (available at local hardware stores or Home Despot). You'll need to cut the hidden part every couple inches to make the curve around the front windows.
Insert nosing is the approx 1" wide plastic that fits inside the extrusions you find back in your bathroom. If the fit is too tight sand the corners on the insert nosing and maybe backside, then use silicone spray to assist with sliding it in. Don't force if still too tight. Getting access to the extrusion end may be quite a job!
Both are available at Oasis RV
in Tucson. Call and talk to their parts department. They are very knowledgeable, helpful and efficient in understanding what you want. Their prices are among the best around. I had what I wanted in Minnesota within 3-4 business days!