Can quite see why 140 AH is inadequate to run the fridge and navigation gear plus lights (and nav lights if night sailing). In UK summer conditions a fridge draws around 4 amps while running, and runs typically for 20-25% of the time, so 24AH, navigation including autopilot is around 5-6 amps, so a day sail another 50AH+, so already over half nominal capacity. Add on other usage such as lights, radio etc of say 10AH and you could well see a voltage of below 12.4v the next morning. An hour motoring will put back between 10-20AH, depending on how well charged the battery was at the start. A day at anchor with nav turned off, but still running the fridge will use probably 35-40AH leading to a flat battery.
If one wants to continue using power like this then the two main things to consider are increasing the usable capacity and increasing the amount of charge. The first is the simplest - doubling the capacity is relatively easy and cheap and then running the engine under load at the start and end of each passage. The latter not only charges the battery but gives you hot water and is good for the engine as running under load (at say 2000rpm/5knots) reduces the chances of the exhaust elbow coking up.
Based on those estimates above around 150AH usable capacity to aim at. The question then is how to achieve that. The three broad choices are lead acid, AGM and now lithium. LA is the cheapest, but in the cheaper bracket of leisure batteries - 2*140s are around £300 but have shorter life, slower recharge rate and higher self discharge rates than AGMs. 3* 95AH such as Exide EK950 cost around £390. Lithium's advantages are based mainly on greater discharge cycle life and deeper discharge so you can greater usable capacity for lower nominal capacity and 2*100AH will give comparable usable capacity to the 3 AGMs plus roughly double the number of cycles and therefore life - but at a cost, currently around around £900 - or nearly 3 times the cost of AGMs.
As I described earlier my boat has 3*AGMs and more than copes with typical UK cruising patterns. Annual discharge cycles are somewhere between 30 and 50, hence my estimate of a 10+year life. As a slight aside, I have a Red Flash 28AH AGM in my Morgan which I bought new in 2003. It is still in use. If I leave the car unused for more than a couple of weeks I plug in the special charger so it is always fully charged. I expect the start and thruster batteries in the boat to last at least as long as this.
Because I have shorepower I don't bother with solar as I just use the mains charger if the battery voltage falls to 12.7v when I get back from a trip, so all batteries are almost always near fully charged when the boat is not in use. However, if my sailing pattern changed and I wanted to spend more days/nights at anchor, a solar panel would help make up for the loss in charge from not running the engine. I really don't think lithium is the way to go as you can't use the benefits sufficiently to justify the substantially higher price. It is different if, like some members here you rely on the 12v supply for continuous heavy use, such as liveaboard when the ability to get greater usable capacity in a given space and a usage pattern that will fully use the extended cycling capacity.
Hope this helps
