Jim,
Are you sure your battery is being fully charged? These guys want about 13.5-13.8 volts for a float-type charge. If you are only getting to 12.5V, the battery won't charge to full capacity. Given GRT's spec of 3.25A for 2 screens + AHRS and the spec sheet for the LR.. battery above, you should be getting a bit over an hour's useful time. Yours should be similar. Of course, that time is affected by charge quality, battery age, number and type of cycles, temperature etc etc. but it seems like you're being short-changed.
FWIW, we change all of the batteries in our computer backup power supplies, alarms etc. every 2 years as a preventative measure (same type of battery). With infrequent use, we find that they age and cannot be relied upon when needed.
Properly used and maintained, these batteries should be a good choice for backups. YMMV
John