For the amount of $$ you'd spend on hotels for two weekends, you could easily *buy* a 0 (or lower) degree bag, and then you'd have it for future use.
The tent heater is probably not much cheaper than a really good 4-season sleeping bag, either.
I've truck camped in temps down to 30 or so, and have found that thick blankets (we use those crazy high-Andes alpaca-wool blankets instead of quilts) are great, but not always sufficient.

Winter backpacking tips I learned:

Dress correctly. Probably goes without saying here but a little cotton goes a long way... to keeping you really cold all night.
I've layered a lower-rated fleece or warm season bag with my 15 degree bag to increase the warmth. Your plan to use extra blankets will probably help.
Don't forget your Thermarest (or equivalent). That insulating layer between you and the ground is really helpful.
Wear a beanie to bed, but not much else. It'll hurt for the first few minutes but a little shivering will raise the temp inside your bag and you should warm up pretty quickly.
Put the next day's clothes in the bag with you. There's nothing worse than putting on cold pants the next morning!
Go to bed with an empty bladder. It doesn't just save that late-night trip outside the tent - the theory is that the less fluid your body has to keep warm the warmer you'll be.
Keep the tent windows cracked. In +/- 30 degree weather your big issue will be moisture. While it seems counterintuitive to leave a window open in low temps, you'll stay drier if you allow the moisture to escape.