It must be the newer trailers and FWs but we regularly got a lot more than one or two days with our battery in our 1993 FW that we purchased in 1993.. We would dry camp, no outlets or water or sewer for four to five days with our initial battery before having to change out for another one. This was before we got a generator. Big difference, I guess, is that we didn't spend a lot of time during the day in the FW, we got out and did things and didn't have an ice maker for the frig or heated door seals, or heavens forbid, a CO2 detector. We had the old regular push and turn light bulbs and would turn off the water heater when we left for the day and didn't have a TV set to run off an inverter. We also turned off the furnace when we were not in the FW as there was no reason to have it running. Also, we turned the furnace down to 55 or 60 when we went to bed and just got some more blankets. Long story short, we have just bought a new FW and will find out how long our batteries will last here pretty soon as we will be going on a little trip in about a month. We talked our dealer into putting two group 27 12-volt batteries into our new FW. Our new FW has a surround sound system with a faceplate that stays on, a CO2 detector that stays on and the frig and furnace. As well, when we turn the lights on in the living room, three of them with two bulbs each come on. It will take us a while, but we will find out how long our batteries last and then, I'll report back.
Early this year I replaced my two 8D gel cell batteries with two 8D AGM batteries. They are rated at 245 amp hours each. Was easily able to run the furnace two nights, along with fluorescent lights, my CPAP breathing machine, laptop and water pump. Those batteries should last 4 to 10 years, hopefully the latter, if properly maintained! I must admit that I was in a bit of price sticker shock for a while, but thats RVing! Oh, those 8D gel cells that I replace were 10 years old and really getting tired, but still going. And they were not treated properly until I replaced the converter charger a little over a year ago.
There is a great deal that can be done to conserve power. I had a great deal of input into the design and what went into our trailer and that included purchasing marine lights that used a G4 bulb now with LED's, the use of computer case fans Espar diesel heater...
You can change things like light fixtures and certainly install switches to turn things off. I bought a Fuse Buddy which can be plugged into a 12 fuse position to monitor the actual use in that can tell me what draws what.
But as pointed out on another forum, while no more than 50% DOD is the ideal to prolong the life of your battery, unless you are full timing the battery will age out prior to suffering damage from the occasional deeper discharge