I just added two of the Die Hard Platinum batteries listed in this post. I have not, however, hooked them up in tandem yet. Spent this past weekend out at the lake, day temps in the 40's, night temps mid to low 20's. Ran the furnace non-stop from 8pm to 8am both Fri and Sat night off of just one battery. Plus lights, but those are now LED, so very small amp draw. Batteries seem to be great, and priced similar to competitive makes/models. My '84 Hawk does not have fridge, still deciding to keep ice box, convert to fridge, or convert space to pantry.
Late September trip was same story, but switched batteries after one night. Ran one battery Fri, switched to second battery on Sat. Once home, hooked up charger, which has % battery life indicator, both batteries at 85%+.