Kevin,
You asked about the 50% draw-down rule of thumb, and how it differs on the lithium batteries. Lithiums (as we'd use in our RV) can be drawn down to about 80% depth-of-discharge (DOD). That's one of their advantages. Off the top of my just-started-my-coffee head, here are the plusses and minuses as I think of them. Note that I'm still on the learning curve with lithium and also lead acid to some extent (but at least I have run those for years).
Lead-acid (I'll use AGM, since that's what I have):
Pros:
Cheaper (ha, they USED to seem expensive, now they are "cheaper").
Can be charged when they are below freezing.
They don't love heat, but they are more tolerant of it, especially if not discharged.
Are damaged by certain mistreatment, but not instantly killed (usually).
Can be read with a Smart Gauge (very accurate SOC).
Relatively low self-discharge rate in storage (especially if not hot).
Cons:
Relatively heavy.
For best overall service life, want to be kept to around 50% DOD*. Hence you are carrying around 50% "dead lead."
Not thrilled with stupid charging.
Less expensive than lithiums, but still not cheap.
Relatively long absorption times use up a lot of the solar charging "day."
Want temperature compensated charging for best accuracy.
Lithium (typically LiFPO4)
Pros:
Can be discharged to around 80% DOD without being bothered.
Lighter (well, you need fewer for same effective amount of amp hours).
No absorption time so all of the day's solar is full-on charging (not voltage limited slow absorption).
Don't need temperature compensated charging (yay can finally use Victron solar controller).
Relatively low self-discharge rate in storage (if not hot).
They shrug off big discharges (no Peukert effect), so running things like a microwave or even air-conditioner can work (of course you have to have the basic capacities and recharge abilities still, but they don't take it on the chin like lead acid - hence why a tiny lithium battery pack can start my V10).
Cons:
Cannot be charged when below freezing (or they DIE).
Lose capacity permanently if kept in hot place (more than lead acid).
Can't be read with smart gauge (minor though).
You thought AGM's were expensive?
(even though you only need about 60% as many)
Wrong charging kills them
Going below 80% DOD can kill them (although battery management system should cut that off).
Need battery management system (BMS) (often part of deal).
Maybe not quite as clear on alternator charging (some BMS say OK, and then limit amount of current that they can take in so as not to suck alternator dry if batteries are low because they can accept a LOT OF charge all at once) (also is charging "good enough" for them from alternator?). Victron does make a BMS specifically aimed at vehicles and alternator charging. They definitely don't want to be charged at 14+ volts, which some alternators put out (no absorb).
I do think lithium are the wave of the present/future, although I can see some circumstances where they wouldn't work. Mainly I'm thinking of temperature control. For example you wouldn't be able to charge them if they were in the Chinook stock exterior compartment and it was below freezing. Also hot weather would be even harder on them than it is on lead acid batteries. I have a small refrigerator in my car (with its own battery and solar panel) and I can't see ever changing that to lithium (as we presently know it) because it just gets too hot in the car. On the other hand that's one of the reasons I moved my Chinook battery bank in under the couch: I figured I'd likely go lithium at some point. Luckily my AGM bank is still doing okay, because it's nice to have some time to see how things evolve.
BG
*PS: The 50% DOD "rule" on lead acid is just a rule of thumb for longest life. There could easily be times when it would make sense to run them lower. Say maybe you can only fit one battery (or you are very weight limited). So your strategy might be to run to 70% DOD and just replace it sooner (but you are only replacing one, and you are carrying around less "dead lead"). The Lifeline Battery Manual (for one example) has a graph that shows the tradeoffs of various DOD % as relates to service life, so you can make a plan. (And if you discharge them less than even 50%, then they last even longer.) So no hard and fast line there.