Watts is watts. It doesn't matter if it comes from a light bulb (of any type), a furnace, a campfire.....or even just rubbing your hands together.
Without calculating your actual heat loss, it's impossible to say, but my guess is that you will need a minimum of a couple hundred watts to even start to be worthwhile doing. That's a whole lot of LED lights and the power usage will be the same as if you used a heat lamp or heat pad of the same total wattage. Watts is watts. If you only want a small amount of heat, then much better (easier and cheaper too) to simply conserve the heat in the water by insulating your pipes and tank.
Haven't seen highs in the low 50's for quite awhile here. Enjoy the warmth.
On edit.
The couple hundred watts I "guessed" at earlier would be to keep your whole basement area above freezing in, maybe, temps down to about zero (maybe?) assuming your basement area was covered and the air was fairly still (not windy). Uncovered and it would be impossible with that small amount of power.
If, on the other hand, you just want to keep a pipe, valve or even your tank above freezing, then it would take much less power....maybe 10 to 50 watts.....maybe....depending on how big the surface was and how it was insulated. Way more than a few LED's will give you, but possible if you use an appropriate heat source (you want something that contacts the surface to be protected....but doesn't get so hot that it melts the plastic).
I still suggest that you cover the bottom of your trailer, and heat and circulate the water in your tank and lines with your existing hot water heater and water pump. Way more possible than trying to generate this power with your limited battery power....and you already have everything you need in your trailer.
