Let us review the bidding again. Don't make this harder than it is - simply proceed step by step
To charge the camper battery you connect it to the starter battery. Literally the positive terminal of the starter battery. Do not disturb the factory wiring between the alternator and the starter battery - you have no idea what is in there.
If the voltage requirements of the starter battery and the camper battery are the same, decide if you want a manual switch, a key controlled switch, or a voltage sensing or "intelligent" switch. Done.
If the voltage requirements are different, you either adjust the voltage of your vehicle (harder) or use a battery to battery charger (B2B) between the starter battery and the camper battery. (Easier) Lots of brands to choose from, each having different advantages.
Now, how do you maintain the starter battery? First, do you have to? Do you drive at least once a month? If so, no problem. Do you park for months in low temperatures? Then you should probably do something. Options include, a shore power charger/maintainer, an add on solar charger, etc.
If you have an intelligent relay AND, there is a separate charge source for the camper battery (e.g. solar or shore), then the problem is solved.
Most (all that I know of) B2B's are one way devices. That is they take from the starter battery and give to the camper battery. So if you use a B2B then you may want some form of bypass back to the starter battery.
-- Sterling Power and TRK-L-START make simple units. There may be a problem with the voltage points with lithium, but I doubt it as AM Solar sells 'em for their lithium setups. Couldn't be easier.
-- Beyond this, we get more exotic. The Magnum SBC could be used as the switch points can be adjusted.
-- Beyond this you can go for a jump start setup.
In any case, there are lots of ideas in the attached image. I use a very complex, Twisted Sister(tm) set up, that connects a 24v starter battery to a 12v camper bank and vice versa. But for the voltage difference, I would simply use a BlueSea ACR.
Enjoy!