The battery dies after two days with the 100w of solar, but you want to last four days and leave with the battery not down too far (50% with FLA or lower with LFP) and do a full recharge on shore power.
So you already know the 100w of solar can't keep up. with one battery, you should be recharging it every day with the converter powered by the generator. (doing 50-80s or 50-90s with FLA, or whatever with LFP) With solar you can reduce the gen time to maybe an hour in the morning and let the solar finish up the rest of the day.
You don't need more battery so much as more charging so it won't die. Your choice of gen times and amount of solar.
If that works then more battery will only be good to have if too many clouds, or your gen won't run. (extra battery is called "reserve" by solar folks)