Waterproofing Treatments for Single Wall Tent

But is it leaking?

Nylon sags when exposed to moisture. It's just one of the limitations.

You could do this, using a dilution of around 4:1 or 5:1 on the outside of the tent:

http://jwbasecamp.com/Articles/Silnylon1/index.html

Edit:
If it's not leaking/misting through, you could give it a spray coating of this:
Atsko Silicone Water-Guard
http://www.atsko.com/shop/catalog/SILICONE-WATER-GUARD-12-fl-oz-aerosol-p-16174.html
(It's generally available at certain large discount stores many try to avoid shopping at.)

No it isn't leaking, just has some slight misting. It is also taking a couple days to dry, where as before it would only be 3-4 hours after a rainy night. I am used to some sag, but usually only after a whole night of heavy rain. This starts to look like that even after a couple of hours of light rain.

Goat

I doubt that you can do much about the sagging issue with any kind of treatment without adding a lot of weight.

What I do with silnylon shelters is to tighten up all lines just before turning in or when it first gets wet from rain. Moisture and/or cold will cause slinylon to expand.

Some people add bungees to their tie-lines to help keep the lines taut. I have tried this a little with good results.

Many threads on this out there.

Or 10:1 or 20:1 dilution – might last longer, probably less weight

I've done it to my silnylon pack and then it was waterproof

But, I don't know that it would cause tent to sag less