ouR escaPOD mods

Ok, I took a look at the battle born isolator. It looks like it tries to keep the things from overheating by switching on and off at 15-20 minute intervals. I’m not sure I could get comfortable with that kind of operation.
Check out this video as an example of what could go wrong:

https://www.victronenergy.com/blog/2019/10/07/careful-alternator-charging-lithium/

My thought has been to isolate the two systems using a dc dc converter with an adjustable output current Then I would set the output current so that the alt didn’t overheat at idle by using my ir thermometer.

The 60A fuse or cb sounds fine to me.

Not sure how the Nissan’s alternator control system works. Most of them are internally regulated to 14.4V max. If your voltage drop is enough on the conductors running to your trailer connector that will limit how much current can flow to the Li battery. If so you might be fine with only an isolator relay disconnect between the trailer and the tv. You need something to keep from dumping the Li battery charge back into the tv battery when the alt isn’t charging, because the Li battery stays at a higher voltage than the lead acid battery does. You could connect up and see what kind of charge currents and voltages and alternator temps you get at idle and at 3000 rpm before you decide what to buy. As long as you keep a close eye on everything while testing you should be ok.

That's good. You still need to be sure you don't overload the alternator when charging, and that the Li battery actually gets charged but not overcharged. I think you should be able to test that manually as long as you're careful. I suggest starting with a partially discharged Li battery and see how much current is drawn at idle and what the temp goes to in your alt.  Then let the battery fully charge and be sure you don't exceed its max voltage rating. If everything looks good then you can probably avoid buying an isolator or dc/dc converter.

The bms will self protect the battery but shouldn’t be depended on to provide day to day charge regulation. Think of it as a backup system. Unlike a lead acid battery, Li batteries will last longer if not charged to 100% SOC, so 14.8V is too high anyway.

Yes, what Battle Born is saying is consistent, you pobably have enough series resistance in the 12V trailer feed so excessive charge current is not a problem. You should also have a 30A breaker in that line already, right? Do you have an ir thermometer to check alt temp while you’re testing?

The 40a fuses will limit the current flow to the trailer battery to 40a if the circuit from the tv goes through them, I’ve never checked to see if that’s the case. But you should still have a fuse or cb in line on the +12V conductor near the tv battery in case there is ever a fault on the conductors to the 7 pin connector. Those are usually 10awg so a 30a cb is appropriate. The aftermarket kits like I have from etrailer have you install that cb on the vehicle body just near the battery.