You can just buy a lithium battery and install it. Will it be truly happy vs a system built around it? No. That said, the existing charging systems in most campers are far from ideal, which is why most batteries AGM batteries don’t last as long as they could.
If you don’t have one yet, I would get a good battery SOC monitor, like the Victron SmartShunt.
Yes, the higher resting voltage on a lithium battery will mean that your camper’s battery will keep the truck topped off by discharging itself via the 7611. Some here like that feature! I means your house battery will rarely be at 100% SOC, and again, some here like that feature!
As for your alternator, if you have not upgraded the standard 10g wiring from your truck to the camper, there is an inline 30A fuse that will potentially trip and protect the alternator. That also means you won’t get charging between them if that happens. A DCDC charger gets around that issue. If you want that DCDC to work well, it will also need fatter wires.
Take it slow on the upgrades if you need to, and if your AGM’s are dead, now is the ideal time to get into Lithium.