Hello there BoHux, Welcome to the FunFinder forum! I'm thinking that you have a very popular model FunFinder and will find the forum awash with great ideas on how to upgrade your 189. Since many of the FunFinder models share components and design features, hints and tips for other models may very well apply!
RE: the tail lights, I believe you are correct about the bulb number (3157). It's late and dark (and a little bit chilly), I'll confirm tomorrow unless someone else beats me to it.
My 2007 x210 developed a tail light issue a while back. I removed the red lense covers to find a bit of a hot mess in there! I cleaned things up (including some corrosion on the bulb sockets) and found that my bulbs were still functional. The problem was that water and other debris had found its way into the housing and caused the problem. Since I didn't know how long the bulbs would last, I decided to not caulk the lenses but did apply some "rubberized" tape along the sides and along the top of the lens where it meets the rear outer wall of my trailer. Doesn't look too tacky and makes for easy access to the bulbs.
If you're not livin' on the edge, you're takin' up too much space!
2007 Shadow Cruiser x210, 2004 RAM 2500 4x4 QuadCab SLT 5.9L CTD, Factory Tow Pkg (3.73 axle), HaulMaster WDH