How do I keep battery banks alive in extreme heat?

Address the enclosure before the battery. VRLA loses roughly 50% of life expectancy per 10 °C sustained above 20 °C, and once heat has damaged a battery the capacity does not come back. In practical order of effect:

1. Measure the internal enclosure temperature in summer, at the terminals. Not the ambient air.
2. Specify temperature-compensated charging. Non-negotiable in an uncontrolled environment.
3. Improve the enclosure. Shading, ventilation, heat-reflective finish, double skin.
4. Then choose the chemistry. LiFePO₄ is preferred where the site genuinely runs hot, because it does not follow the lead-acid temperature-life curve.