ouR escaPOD mods

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 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.

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 rule with fuses and circuit breakers is to always size them smaller than the max current rating (the ampacity) of the conductor that the fuse is protecting, but at least 25% higher than the max current you will ever normally draw through that conductor. The 25% is to avoid nuisance cb trips or fuse blows.

In your case, the ampacity of 4awg 105C wire is 95A so that sets your upper limit for fuse selection. You’ll need to set your lower limit based on what you plan to hang on your 12V buss, but if you just have the normal RPod dc loads we know that 40 amps handles them fine. So 50A is probably your lower limit unless you’re adding an inverter or something.

So any 12V rated fuse or cb between 50 and 95A will work. Personally, I prefer to stay at the lower end of the allowable range, I’d rather have a fuse blow sooner rather than later if there is a problem. If you get a fuse rather than a breaker be sure to order a spare.

I’ll take a look at battle borne’s battery isolator.

StephenH - interesting mod. One thing that I am not clear on is the reasoning for going in this direction. Seems like a pretty hefty investment; what are the benefits that you are expecting?