Tom,
Lead acid batteries have a rated life cycle, so partial or mini cycles will, to a degree, provide a bit longer calendar life, but total cycle life still adds up to be the same, unless batteries are abused. Meaning, they are left partially discharged for 5 days or more, some times weeks. That kind of abuse will shorten life quite a bit, but still less so on AGM batteries. People have been greatly misled by misinformation, wives tails/myths, that if one exceeds the magical number of 50% SOC their batteries will be irreparably damaged, the 50% mark is an ideal guide/goal, but not a must. True deep cycle lead acid batteries are designed to be able to handle up to 80% depth of discharge, meaning, a SOC down to 20% without damage. The key thing is to bring them to full charge at least every other day, preferably every day and not allow them to sit at that low of SOC for longer than a day. This is especially a non issue if these conditions happen only occasionally, that is why that cushion is designed in by the battery manufacturer. Believe me, these deep cycle golf cart batteries endure much more abuse in golf carts, being run flat multiple times a day and they still hold up and serve the industry very well. Some AGM battery manufacturers allow and/or recommend a mini/gentle equalize cycle (might be something worth checking into). If deep cycling is required and the manufacturer allows for gentle equalize cycles, it would not be a bad idea to equalize them every now and again, when there is plenty of sun, further prolonging battery life. This additional capacity, that people are lead to believe that it is not allowed to be used, would likely give you the needed amp hours to get you through the night, and allow you to give a good bulk charge in the morning with the generator, then topping off with solar. This additional cushion will very likely eliminate the need to charge at night, when it is most wasteful. Of course, worrying too much about usage and state of charge is not only stressful, but can drive your wife and yourself batty.
So testing the system while having the safety cushion of campground power is ideal. To simulate winter conditions, you could even turn your solar off around 3 or 4pm and see how many amp/hrs you draw overnight. I would not recommend doing an amp/hr test if it involves running the batteries flat. If your meter is off by a little bit, it's not a big issue. What brand is your shunt based battery meter?
As for the refrigerator, the defrost timer is not hard to find. It is either located under the cover of the cooling temperature settings or the front bottom section of the refrigerator, where the air intake is for the condenser coils and compressor.
The timer should look similar to the one in this link:
https://www.amazon.com/Refrigerator-Rep ... 5987&psc=1
The circle in the middle is the clock, just turn it until you hear a click and you will see the amps rise, do that around late morning. I have seen on some refrigerators where the timer is located by the temp dials, where it has an access hole to be able to adjust the time w/o taking the cover off.
If you don't feel comfortable with making those adjustments, you can choose to set your timer to go on for 20 to 30 minutes, once or twice during the night. That would be plenty to keep things cold, as these new units are very well insulated.
Have you eliminated phantom drains like the sound system, microwave, etc.?
For an example, my microwave draws a whopping 7 amps just sitting idle, doing nothing. I have connected through a power outlet bar and switch it on and off as needed, same with the TV/sound system. If you don't want to fuss with turning those items on and off, you could opt to put them on timers to go off around the time you retire to bed for the night. These are all various ideas to consider if you have a lot of phantom drains, and they do add up.
As for the Olympian heater not keeping up. Have you considered an electric blanket for your wife? They don't draw that much, especially the 12v units. Once your warm, it can be dialed down to a low setting, so the power usage is minimal. Saving on power and on pricey diesel fuel.
As for battery compartment. I know a few members who have built a battery tray in the engine compartment and moved the engine batteries into the engine compartment, adding two additional batteries for the house bank. This option would allow you to have 600amp/hrs of lithium if needed.
Also, have you considered locating the lithium batteries in the rear section of the basement?
This would allow the batteries to stay above freezing, giving you peace of mind and keep them cooler in the summer. It would also allow you the flexibility to get the low cost, but high quality EG4 server rack batteries installed. Each battery is 400 amp/hrs for $1500, so for $3K, you could have 800 amp/hrs and get rid of all your worries. BTW, those server rack batteries just got their UL listing, so good bang for the buck. If your inverter is strong enough, with 800 amp/hrs, you could run one a/c unit during the day, with long summer sun hours, you should have no issues.
All ideas to further muddy the lithium waters.
If you do intend to keep the batteries in the exposed original battery compartment, make sure that the battery brand that you do go with, truly has low temperature cutoff for the battery charging (many brands claim they do, but actually don't), otherwise irreparable damage can occur if the batteries are charged bellow freezing temps.
Something else to consider if the lithium batteries are exposed to the cold elements, even with cold weather protection and pad heaters. If you happen to get caught on one of those polar vortexes, like people did in TX a couple of years ago, it might now be enough to keep your batteries warm to allow a charge, potentially putting you in a difficult situation. So maybe insulating or enclosing the battery compartment or getting some kind of thermal blanket for those extreme weather conditions that you may bump into. This way the pad heaters could keep up if it is very cold and windy to boot.
What brand and model charge controller are you using?
What wattage solar panels do you have, and are they connected in parallel or series?
During peak sun, under peak load, what is the maximum amp output from your solar charge controller are you seeing on your battery meter?