Thanks for the comments. Two of you had batteries last 10 years so maybe I can chance it.
My concern is that it will be a major issue if my batteries fail or cannot keep up with my use. I would likely need to drive hundreds of miles to get to a dealer since I will be traveling mostly in the Canadian Rockies, at Olympia NP and in southern Utah. Dealers rarely stock the GLP 30HT batteries that fit my compartment.
As far as a load test goes a heater will not work. First my electrical system does not have an inverter. I do have a small inverter for charging camera batteries. My system would not support an inverter big enough to handle a typical 120v space heater. A small space heater pulls about 12 amps AC which is 120 amps for my 12v DC system. No way. But thanks for the suggestion.