[JRTJH]

I don't think stuffing fiberglass in a small area around that cutout will do much good. The lines most likely run from front to back in that underbelly, stuffing insulation around a 3 ft area close to that hole won't do anything for the remaining part of the underbelly that isn't "stuffed" so, I'd suggest not wasting your time.

Once you get the skirting up, if the light doesn't provide enough heat to thaw the frozen pipe, you might try putting an electric heater under there for a few hours. Just be very careful and watch closely that it doesn't get too hot right above the heater.

As Festus2 said, pipe insulation will help if you can find the pipes to get the foam around them. Another use for those foam pipe wraps is to stuff around the slide flanges inside the RV. Those seals are not the best for keeping cold air out, any extra soft foam rubber you have that you can use to help block the air infiltration around the slides will help trememdously. Don't forget to lift the carpet covering the floor of the slide and slip some foam in there too. The floor may be a little easier to access from outside, but you'll find air leaks top, bottom and both sides of almost every slide.

As for your frozen pipe, I'd still look inside the trailer, next to the wall, inside cupboards. You may find that the pipe that's frozen is inside, next to that outdoor shower fixture, or very close to it. Opening the cupboard doors, using a small fan to push warm air through the cupboards will certainly help thaw anything in that part of the RV. If you haven't already, you might want to put a light in the pass through under the bathroom, or even a small oil filled heater if you have access to one. Use it for a few hours to warm up that area and you might thaw the kitchen pipes. If you do decide to use a heater there and also inside the RV, probably would be best to run an extension cord from the park power pedastle to the one in the pass through. That way you're not increasing the load on your 30 amp circuit inside the RV.

As for windows, you can put plastic sheeting over them, both inside and outside to help slow down the heat transfer through those single panes of glass. If you haven't already, look up at the vents in the ceiling. They are a big "cold spot" in any RV. you can buy pillows to fit those openings and insulate them when not being used. You can also cut a piece of foam board to fit them for much less money. Even though it's only R5, that's much more insulation than the plastic vent cover provides.

Realize, you've got around an R9 floor, R7 to R9 walls and an R 9 ceiling, single pane windows, almost no insulation behind the refrigerator, and air leaks virtually everywhere, so you're not going to make it as "snug" as a stick built house, but you can make a huge difference with window coverings and skirting.