Heavy duty outdoor thread

I'm building a Yurt and so far have done pretty well with sourcing natural materials. I have 12z duck cotton canvas for the cover, and am looking for thread to sew it with. The instructions I'm following call for #69 bonded nylon thread (for the 10z duck Canvas they use). How would mercerised cotton perform by comparison? Would I use the same weight? Is there a better natural fiber I could use?

I used Mercerized cotton thread like for jeans for my outdoor tent.  I figured the fabric was cotton so it would likely last the same amount as the thread.  But I did french seams for all mine (so I didn't need to stuff the fabric under the arm of the sewing machine) which has a double row of stitching for each seam to make it stronger.  

I haven't left my tent out over the winter but it does travel a bit.  

I went to a sewing specialty store yesterday and the saleswoman explained that mercerized was just done to reduce lint, the uv resistance would be the same as regular cotton tread. I'd like to know what ballpark uv deterioration would actually be in, seems we've mostly been brainwashed by consumer society to fear such uncertainty and automatically select "obviously superior" products from the petroleum industry without question.

My Yurt plans call for a waterproof stitch that is double sewn, but doesn't fit neatly through a machine like the French. How does the French one do for water resistance?

The french seams are as water-resistant as the rest of the cotton duck cloth.  Fine under moderate to high tension, stupid if you have something pressing on them.  

I don't know if you are old enough to have one of those cotton tents as a kid and have your parents yell at you for touching the cloth.  It's like that.

Generally, I like the thread to be equal to or weaker than the fabric as a design... like how a machine is designed for the cheapest and easiest part to fail.  A seam is easier to fix than mending a tear in the cloth.