If you are skirted with straw you could place an electric heater under the trailer under the bathroom area. Probably not a good idea with straw bales tho. I would do as Crabman says and turn up the temp.
2011 Eagle 330RLTS with just about every option.
Dodge SRW 1 ton Cummins powered.
I was born with nothing and I still have most of it left.