Keep your batteries at 85% or more and you are good to -67 per Trojan. See http://www.trojanbattery.com/pdf/WP_DeepCycleBatteryStorage_0512.pdf
Instead of a dashboard unit consider a real solar system for camping and storage.
Winnebago 2101DS TT & 2022 Chevy Silverado 1500 LTZ Z71, WindyNation 300 watt solar-Lossigy 200 AH Lithium battery. Prefer boondocking, USFS, COE, BLM, NPS, TVA, state camps. Bicyclist. 14 yr. Army -11B40 then 11A - (MOS 1542 & 1560) IOBC & IOAC grad