I've done this job twice, including replacing two brittle mouldings just today. Most T-mouldings available are lipless and so aren't great for edging walls in an RV. With the exception of one I just found at the West & Barker link previously mentioned (thanks for that, by the way), I have been unable to find lipped T-mouldings anywhere near the width I need for my RV wall edges.
I've just been resorting to using U-mouldings of the proper width, with Liquid Nails spread inside the mouldings, and they work fine. I get mine from these folks, which is somewhat handy for me since one of their two HQs is within driving distance of my home. They stock white and black in 12' sticks, and if I need a color (today I needed something akin to "peanut butter"), I just paint the white ones.
The styrene scribes really easily with a Dremel sanding wheel (curves) or hand file (slots), to accommodate sink and tub edges, shelves, and so on.