Im quite surprised that the jrc or the tracker bags dont keep you warm especially if they are 5 season but bags will lose there thermal proprieties over time so if they are both quite old then that could be why.
For a new sleeping bag, you have two options.
First is to buy a bag made by a tackle company, probably the most comfortable since they will normally have straps on them to attach them to the bedchair so you dont have to struggle to get out when you have a bite. I personally use a the Total fishing gear 5 season bag and has done me well in very cold temps as i use it for camping as well. They also do a beefed up version which is even warmer.
the second option is to buy a sleeping bag from a dedicated camping company which will just be a normal sleeping bag but you will find they have been designed with very cold temps in mind.
Plus you could look into getting a bedchair cover which will help keep you warm in the colder months.
other things you can do to keep warm is have a clean set of clothes to change into when your getting ready to get in the bag. These will be dry so will keep you much warmer, since sweat will stay on your clothes and get cold which will make you cold.
Put your unneeded tackle bags under your bedchair and this will help keep the cold air from the floor coming onto your bed.
and take a blanket, This will give you a extra layer of warmth.