You're right on! I've thought about that quite a bit. There is an argument for buying the cheapest decent wet batteries, taking them way down and replacing them often - you're absolutely correct that it's simply a cost vs. usable capacity issue. Another thing you can add to your argument is that 3000 cycles is a lot of cycles - unless you keep your RV a very long time and use it a lot, you may never get to the end of life for lith phosphate batteries.
There are certainly other issues to consider. Weight is a big one for me, because I have an MB - by the time the two of us climb in with our belongings and a few bicycles, there's very little cargo capacity left - an extra 100 lbs is extremely desirable.
Another issue is voltage drop as the % SOC drops. I'm looking at charts for one of the Full River deep cycle 6v batteries - that must be similar to what you have. The voltage vs. SOC chart shows ~12.8 at 100% and ~11.75 at 20%. That's usable but getting pretty far down there. I'd expect my batteries to be ~13.4 at 100% and ~12.8 at 20%. So the lith batteries have less voltage drop and higher voltage, both of which are advantageous. Less current to run a device that requires a given wattage, if the voltage is higher. These numbers are rest voltage, not voltage under load.
Full River:
http://www.fullriverbattery.com/product/batteries/DC224-6
There's a further issue, though I'm not taking advantage of it. Lith batteries can be both discharged and recharged at high rates without hurting the chemistry. They'd be good batteries for an RV with a 2nd alternator dedicated to battery charging. There's been quite a bit of discussion of that on this and other forums as an alternative to solar. If I can install a 2nd alternator dedicated to battery recharging and capable of 160A charging rate, I can fully recharge in an hour, and the price is similar to what I was seeing for a 400W solar installation. And I don't have to park in the sun with no A/C on <g>...
Added to that, lith recharging is simpler than flooded cell recharge - no multi-mode chargers that bulk charge fast and then take a long time to get the battery fully charged. You use a CCCV constant-current constant-voltage charger that simply puts out its rated current - and the voltage climbs as the batteries get near full charge, until it's up to its rated voltage, 14.6 in the case of my charger. Then the charger remains at 14.6 and as the battery approaches that voltage the charger current drops to near zero. At that point I'm fully charged - if I remove the batteries from both charge and load and let them sit for a day, they'll drop to 13.4, their nominal full charge voltage. There's a possible issue concerning whether lith batteries can be damaged if you leave the charger connected once they're at 100% SOC, but to complex to go into here, and there's no consensus at present on it.
The last issue that comes to mind is shelf discharge. Much of the discussion on solar has been about small solar installations that'll keep the batteries topped up when the RV sits for a month or so. There's less need for this with lith batteries - they don't self discharge very fast, and they don't care (in terms of their chemistry) whether they're sitting at 80% or 30% SOC. So I've just parked my RV in a storage facility for the winter (in VT - long winter!) and I'll probably get it out early in April. I'm not trickle charging, and I'm expecting to find the SOC still pretty far up there next Spring.
The biggest concern for me at present is temperature. Lith batteries can be discharged below freezing, but they don't like to be charged below freezing (battery temp, not outside ambient temp). That Full River page says their battery 100% of rated capacity at 80 degrees F and 76% at 20 degrees. My lith batteries are 100% at 80 degrees and ~82% at 20 degrees - a bit of an improvement. But I can damage the batteries if I charge when they're below freezing, so I need to either remember not to let them charge or provide dircuitry to do this - working on that issue right now! It's easy for the charger - I can simply turn it off by throwing its circuit breaker. But the alternator is wired directly to the batteries - turning off the DC switch doesn't prevent the batteries from being charged by the alternator. So there's some rethinking and rewiring needed there.
Those are most of the issues that come immediately to mind.
Later - another thought. When I read your post, I thought that your life cycle figures were optimistic for both flooded cell and lithium - I'd have to go back to my spreadsheets, but my financial calculations were based on 600-700 cycles at 50% discharge for wet cell and 2000 cycles for lith phosphate. I think the discussions about lith batteries have made the manufacturers and sellers more aware of this issue, and I noticed today that the life cycle figures for both types are more optimistic than they were a few years ago when I started pushing a pencil and comparing.