The 28A is a very nice design -- I like it a lot -- and was glad to learn Thor is still making it even though it's not listed as a floorplan. It's a little too large for just the two of us, but if I planned to travel with others on a regular basis, it would be near the top of the list.
I'm sure you've already checked the obvious like vents being open, etc. so there is little I can add. My only suggestion is that A/C capacity in an RV is ultimately shared throughout the unit, so if there is anything you can do to reduce cooling load somewhere else, it will benefit the bedroom also. One of the highest contributors is solar coming through windows and windshield, so parking in shade or with most windows out of direct sunshine is helpful. On our rentals I covered the windshield to reduce cooling and heating loads; plus for privacy.
Another suggestion is to do a quick A/C cooling test to confirm your A/C is operating properly. Temperature readings in and out of unit can show whether it's operating as designed, or whether it needs freon or something else.