32 year old male. About to go camp in two weeks, expected temperature -5 celcius. Spent 3 nights in August in a decently expensive synthetic bag, around +4 celcius comfort and was uncomfortably cold every night (generally cold human being). Mostly going to camp during the summers, but also taking it out in -5/-10 celsius aswell. Rain is quite common here but will rarely camp for more than 2 consecutive nights. I know there is quite the difference between the three bags, but I can get them for the same price. I guess weight is somewhat of an issue, but not really, comfort and warmth are prioritized.
Keep your synthetic bag because it dries well. Add a liner for the cold temps. That gives you the best of all worlds - take the liner when you need it and save the weight when you don't. Marmot makes some good stuff.
You could also just use your current down bag inside a cheap wide fitting rectangular bag. My own system consists of an ultralight PhD down mummy complemented in winter by a wide outer bag with their dryshell fabric and an artificial fibre fill. This works very well. And remember, however good your bag & sleeping mat you will not be warm unless you bed down with (preferably hot) food & drink inside you. A sleeping bag will only conserve the heat your body produces from the fuel you provide. Crawl into a sleeping bag feeling cold & hungry & you can more or less guarantee to stay like that all night.
Welcome KeeleyKelley to the forum. A lot of good info is here relating to RVing and enjoying the great outdoors. We look forward to your contributions to forum discussions and reviews of campgrounds you have visited. When posting to a forum thread keep in mind the age of the thread. This one, for example, is almost 5 years old I doubt you will receive many comments to your post.