We could answer better as to the size if we knew what size RV you plan on heating with it.
I'd say either the 6000 or 8000 btuh unit would do what you need. Since there's no thermostat on those units I'm not sure if the 8000 btu unit would be too big. But, there's only $20 difference between the 6000 & 8000 units.
P.S. Yeah, I went to your profile and saw that it's a 23IKSS, but it would be nice to put that in your OP.