You're right, the problem with heating the environment is that it can produce "heat waves" which show up in your photographs if you're not very careful. All in all it seemed like bringing a heater (propane, or otherwise) was something that I was going to have to monitor and fiddle with a lot, so I didn't ever go down the path of trying to keep the environmental temperature above the dew point... One of my goals when doing time lapses is to be as automated as possible, so the heater didn't 'pass the test' for me in that regard.

As you might imagine, this is a very common problem in the astronomy world, so it's one area where we can simply borrow liberally from their techniques (and their equipment as well).

1. Lens Sleeve: On the 'low cost' end of the scale is simply wrapping the front part of your lens. I went out and bought some neoprene and fashioned my own 'lens sleeve,' but if you're more interested in purchasing something, telescope accessory manufacturers such as Orion make similar products...

2. Heat Strap: One step up the scale (in both sophistication and complexity) is to use a heating strap. With a heating strap you simply strap it to your lens near the front element and it keeps the glass warm enough that condensation can't form.

3. Multiple Solutions: If you're expecting really wide temperature swings (depends on your region... Where I live in the spring and fall we can experience 20 to 30 degree swings between day and night), a single heating strap may not be enough. I've had some success with the lens sleeve plus the heating strap, but I finally did bite the bullet a couple years ago and bought a multi-channel prevention system so I could keep the entire lens warm (not just at the front element). Carrying the idea to its extreme, I've recently done some full-winter day-to-night timelapses here in the northwest where I've been using a full neoprene 'suit' for my entire camera and lens (made myself, cut the pieces from an old wetsuit and sewed them together like a 8-year-old cub scout trying to earn his leatherwork badge at camp! It ain't pretty, but it works...) and a multi-channel heating system together, which worked like a champ. Obviously the downside to this type of solution is its complexity, but for all-night timelapses I'm already hauling car batteries and motorized dolly platforms, so adding another powered item didn't seem like such a big deal. :-)

(NOTE: I'm not affiliated with Orion Telescopes, and there are other companies that manufacture similar products... Just a user of their products.)