Load Center - Circuits allowed/ can I have more tandem breakers

I have a van that I'm working on as a small camper project. I want to leave the stock battery for vehicle startup, and then add an additional battery in the cargo area to power various things while the truck is not running. Some of my questions and concerns are these: 1) I do not plan to run the batteries in parallel, so do both batteries need to be the same make/model and specs to work properly? Can I just leave the stock battery as it is and then buy a new deep cycle battery for the cargo area and then not have to worry about it? 2) The way I intended to wire them up was to leave the stock battery as it is, and then run the positive of the extra battery to the alternator and then give the extra battery it's own ground in the cargo area. Then I would wire and power whatever I needed to power from that cargo area battery and not have my startup battery touched at all. Is that going to work well just as I've explained? If not, what am I missing? 3) Is my stock van alternator going to have any problems with the added battery and keeping both batteries charged? Do I need to upgrade the alternator so that there are no issues? 4) I've heard some people talk about the need for a battery isolator when running dual batteries, is this a case where that would be needed? If so, please explain. If you need any other information from me, feel free to ask. I have done a bit of research on this, but I am still not entirely sure of if my ideas about this is correct. I can use some guidance from those who know about this stuff, so thanks to those who reply.

I'm trying to add a hot tub near where the A/C has a sub-panel. On the main there is a 90 amp breaker. Wire to the sub-panel is 2 hot and a bare aluminum neutral. Looks like it is a 3 wire feed, not a 4 wire. The sub-panel has a 2 pole 40 amp breaker for the A/C and an extra 60 amp breaker that isn't attached to anything. This is an older house, built around 1985, so I guess they weren't required back then to run a 4 wire feed. I need to see what the wire size is for the sub-panel but it looks a lot bigger then 8. Right now there's a dual unit A/C and propane heat. Maybe at one time there was also electric heat. But in any case, it looks like my feed is big enough to handle the addition of the spa. The spa can run either 120 or 240 and the maximum amps needed are 40 with quick heat configured. The manual says even when configured as 240, the electronics run on 120. So my question is - do I need a ground? If so, can I just add a ground rod and isolated ground bus in the sub-panel? I think you can do this for separate buildings. Does the same also for older construction where you only have a 3 wire feed. And if I add the ground rod, can I then have a 120 GFI receptacle. Thanks for any help.