I have a 2012 29X and I know exactly what you have--Just like mine.
According to the plumbing prints supplied by Thor that IS the only LPD. Mine is the same way, about 1- 1 1/4 inches left in Fresh Water tank. Seems also I read/heard somewhere that in a 29X, the outdoor showers (I have two, one below the kitchen sink on the outside and the other one under the bathroom sink next to the toilette on the outside) are also low point drains for their respectable portion of plumbing. Seems reasonable but not sure.
Being in Florida I have never have to winterize mine, but I thought about it last November when we were in MI. but I lucked out. (headed south in time) I kind of figured if I had to winterize I would find a way to get the rear of the MH up high as possible and open the LPD to get the water level below the FW tank plumbing, drain the water heater, and use my air compressor to blow out all the lines. I would leave the remaining water in the tank for the winter. Not sure but I think I would probably leave the LPD open including the outdoor showers. The important thing is draining all the plumbing lines, they will split if they they have water in them---the inch of water in the tank I assume should be able to expand.
Only my opinion, Your results may vary.
BTW--How do you like your 29X? Not a whole lot of them out there and there seems to be a couple different varations of couches/chairs. DW and I enjoy ours. Unfortunitly I lost a bedroom slide motor last Thursday and trying to get fixed (warranty) before we take off again on the 11th for 2 weeks.
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