Concept for cold weather use of rPod water systems

texman, you have the operating theory correct, Just one clarification,  the thermostat on the water heater operates independently of the one on the recirc system. The water heater will turn on when its thermostat calls for heat, which might or might not be when the recirc system is operating. 

The purpose for adding the new thermostat on the water heater is only to be able to set the water temp lower than the 130 deg non adjustable setting of the standard thermostat. By setting the temp down around 102-110 degrees (whatever feels comfortable to you) you don't ever get scalded, you don't need to fiddle with adjusting the water temp, and you waste less water. So, you won't need to change the recirc system back to its normal hot and cold setup in order to use the water in the winter. 

In my case, when I do the recirc mod I don't think I'll ever change it back to allow separate hot and cold line function, even in summer. As I don't drink or rinse food with the trailer tap water (I don't trust the water in the tank so I use bottled water for those purposes and for brushing my teeth),  I really have no need for the cold water function. 

I think you shouldn't need to set the recirc thermostat setpoint anywhere near as high as 60 degrees. If its temp sensor is located at the coldest low point of the system its probably OK to set it more like 35-40 degrees. Some testing will probably be required to dial that in. The temp sensors on the little thermostat boards are thermistors so if the wires to them are too short you can cut them and splice in longer wires without causing a problem.  The boards are supposed to have a temp calibration adjustment but the one I bought didn't, so I had to use a separate thermometer to set up mine the first time. Just immerse both the temp probe and a known good thermometer in a glass of water and record the temp difference for later use. 

Re your earlier question about whether the water recirc freeze protection will work for the risers up to the trailer appliances from the water lines in the floor, I would assume that it doesn't until proven otherwise. So, there should be a way for maintaining the interior of the trailer above freezing at all times unless you winterize. I'd also leave the cabinet doors open a bit under the sink and by the new cutoffs you would install for the outside shower. 

That being said, since those appliance lines are risers, it is very likely that the water in them will warm somewhat just from natural convection from the heated lines in the floor. You'd have to experiment a bit and maybe take some temp measurements to determine how warm the interior of the trailer needs to be to keep them from freezing. It might be a pretty low temp.