Once upon a time I lived in Iowa and built a greenhouse. I had a choice between propane and electricity. I chose propane.
I chose poorly. Go electric. Propane is an annoying chore for a greenhouse. Even though I went with a 30 pound tank to reduce the frequency of trips, and kept the greenhouse super-cold (wasn't growing true tropicals back then, just "garden plants"), the periodic replacements were a drain on my life. Also note my caveat further down in this post about propane heat. Now, you're certainly in a warmer climate than I was in Iowa! But you're also surely not wanting to keep your greenhouse super-cold either.
Now, when I say "go electric", am I saying "add electric heaters"? NO! Add electric lights! Your plants will lack light in the winter (even if you do what I did and put reflective insulation on the north walls); mine sure hated it. Ideally go LED if your budget supports it; even with LED, most energy that goes in is wasted as heat, but you get a *lot* more light out than you do with incandescent for a given wattage.
Do black-painted water barrels help? A little. But not as much as you'd hope. Also note that the water inside will steadily get ucky, and if you add chlorine, it'll shorten the barrels' lifespan. Better for heat storage would be - if your budget allows for it - running tubing underground and hooking up a blower (or a water system, so long as you're sure it won't freeze). I had an idea for a "low budget" version at one point involving an auger and doing pairs of 45° holes that meet at the bottom, then fitting PVC tubing through them forming a zigzag network. But I never implemented it.
Some other things:
Do yourself a favour and run water to your greenhouse; hauling buckets of water is tiresome work. And don't do what I did and manually dig a trench with a garden hose in it; get a proper trenching tool so that it'll be deep enough to not freeze, and have a plumber connect a water line to the *inside* of your house. Or if you don't want to do that, then find an electric heating solution for where the hose is exposed outdoors on its way into the ground. Insulation alone isn't enough to keep that from freezing. And on the greenhouse end, make sure that the hose comes up *inside* the greenhouse, not outside as I foolishly did in my first iteration.
You can set up an automatic watering system to save you from having to go outside in the cold, but be careful. You have to keep your electronics from getting wet first off. But secondly, don't make my ultimately fatal mistake. I had a painted-steel framed kit greenhouse (which I heavily adapted over time). My watering system *dramatically* shortened its lifespan, and ultimately rusted it away after just several years. The thing actually collapsed on me while I was inside. Now, my heavy insulation that I added on the north wall sure didn't help things any (lots of extra weight), but it was fine until the steel rusted; by that point, a windstorm would have taken it down regardless even if the insulation weight wasn't there.
If you don't have them, install vent openers and/or fans. Leaving the door open all the time in the summer may be fine, and always closed in the winter, but in spring and fall it's a twice-daily task of opening and shutting them manually. And if you screw up? Hello scorched plants. Or more accurately, goodbye scorched plants. I did implement an improvised solution that did actually work (solar panels hooked up to fans on a small vent in the roof - it vented when the sun shone, and stopped at night), but a proper opener would be better.
Oh, remember that caveat about propane heat that I mentioned above? Here it is: the heater Isaac linked is NOT AN INDOOR HEATER. At least as far as I can tell from the ad.
It could kill you if you use it in the enclosed space of a greenhouse. And FYI, your plants can be poisoned by carbon monoxide as well. You need an *indoor* propane heater. There are two types, indoor-rated unvented heaters, and vented heaters. The former - even though its rated for indoor use - will still worsen your air quality (buildup of ethylene is the biggest one for plants). And if the greenhouse is too well sealed, it'll keep hitting its oxygen cutoff. I used an indoor-rated unvented heater because I thought the extra CO2 would be worth it, but it wasn't. I'd recommend an outdoor, vented heater if you go propane (or natural gas).
Concerning indoor air quality: you may want to seal every gap, but greenhouses need to breathe. Even without a propane heater, an overly sealed greenhouse will suffer from ethylene buildup (plants give it off on their own), and ethylene is toxic to plants at much lower levels than carbon monoxide is to humans. One of my DIY projects which I *was* happy with was my DIY heat exchanging ventilation system, which brought in fresh air while heating it up with outgoing air.
Ultimately, though, after my greenhouse rusted, I didn't rebuild. Since I no longer had a greenhouse, I began starting my seeds indoors. And they kept taking up increasing amounts of space. And needing more and more light. And then it got to the point of, "hey, I have all this light here, I can grow plants that I never would have been able to otherwise, and just never move them outside". And so bit by bit began my bad habit of obsessively culturing exotic tropicals indoors