There is some great info on this thread. There are just a few key details missing, so I'll try and fill those in:
When computing the usage of an appliance like a fridge, you have to consider the "duty cycle" - the amount of time that the big loads like the compressor are actually running. This is a pretty good writeup on a Norcold NR751 that is the size typically installed in vans like ours: https://www.buildagreenrv.com/design...ectricity-use/
. Considering duty cycle, the the NR751 average use is 2ah @12V.
On a 200ah AGM battery, you can use all 200ah, but if you repeatedly use all 100% of the capacity (Depth of Discharge/DOD) you reduce the useful life of the battery. Top quality battery vendors publish curves that show how battery life varies by DOD. Here are the stats for our AGM batteries: 700 charge/discharge cycles @ 100% DOD, 1,600 cycles @ 50% DOD, 3,000 cycles @ 25% DOD.
One of the big improvements with lithium batteries is you can use 100% of capacity with a minimal battery life reduction. So, a lithium battery might allow 3,000 cycles @ 100% DOD.
For either AGM or Lithium, if you charge or discharge the battery at currents that are too high, bad things can happen. The battery's pressure relief valve vent might open discharging fumes and causing a rapid capacity loss. You might blow a big, expensive and hard to replace fuse. You might melt something or start a fire. You could cause your chassis to malfunction if you pull the chassis voltage too low.
Lithium batteries can be damaged by charging below 0F (exact temperature varies) or overcharging. As mentioned above, currents that are too high can also cause damage. A typical BMS can't really solve these issue, but it can disconnect the battery if it detects that the battery is being operated in a way that could cause damage.
Both AGM and lithium batteries have reduced capacity at low temperatures. High temperatures can cause other types of problems.
To get a complete charge, both AGM and lithum batteries need to be charged with a "charge profile" that matches the battery design. If you use a non-ideal profile to charge your battery the results vary from suboptimal battery capacity to battery damage. Lithium batteries need to be balanced to get a complete charge. The balancing function can be built into the BMS or it can be a separate device.