While using voltage readings to determine the state of charge of a battery is possible, there are a couple of things to consider.
First, after charging, wait at least 15 minutes, (1/2 hour is better) before checking the battery. A surface voltage reading immediately after charging is misleading. A fully charged battery should be around 12.6V so i suspect you are not fully charging the battery with your hour and one half charge.
Another point - reading battery voltage while under a load (for example, while the furnace is running) will also be inaccurate, so I would not be concerned about 11.59V while running. A more accurate state of charge is the morning reading of 12.28. That indicates about 65% full battery, another indication that you are not fully charging the battery.
What are you using for a charger? At 13.67V, it will take a long time to fully charge your battery (that is many hours). It would be better if your charger can produce a "bulk Charging" voltage in the neighborhood of 14.5 - 15V if you expect to get a full charge in a couple of hours.
A good, 3 stage converter will charge at bulk rates until the battery is close to 80%, then switch to absorption levels of 14V or so. When fully charged, the converter switches to float levels at around 13V. While the WFCO converter used in my Escape 17B is described as a 3 stage converter, it has a reputation of not ever going into the bulk stage unless the wiring between it & the battery is short & large. Again, your charging voltage measurements indicates is is not going into bulk charging which is why you are not fully charging the battery.
One last point (added on edit) - You are comparing your 105 amp/hr battery to a pair of 6V batteries with over twice the storage (232 amp/hrs). Still, if you cannot fully charge your battery after 24 hours on the converter, it is time to look for problems with the battery itself. Some possibilities include a bad cell, battery sulfation
, etc. Unless mistreated, a 2 year old Trojan battery should still function.
Here is a link
to a printable "Voltage / State of Charge" table that is from the site linked below.
Lots of good information at Mark's "The 12V Side of Life