question

BMV. The shunt goes between battery and ground. With a light connection to positive for volts. Any loads that bypass the battery will not be recorded.

All the other components work together, but check panel voltage for your panels and intended wiring. if you go with two panels in series now, then add another series pair later, you can probably use a smaller MPPT.

The Orion's are one way, and have voltage sensing/engine detection built in. You can download the manual, it describes very clearly how this works. You may need to adjust some settings.

@kevgermany Right, I saw in the BMV-712 installation that the shunt goes on the battery negative between loads/chargers. So it'd sense any of the SmartMPPT, Orion, Blue Smart, right? That'd be great for me.

Reading closer in the manual on the Orion, I see what you mean, nice to see they have advanced engine/alternator on/off stuff. Perfect!

Reading through the Orion and Smart MPPT manuals closer, they talk about fusing between it and the battery. Is there a particular type of fuse (I know they have different blow characteristics) I should use? I was planning on using this style (not necessarily that brand/model) breaker with temperature disconnect, between all the components to serve as both fuse/breaker & cut-off switch as needed. And then also 12v fuse block for all of the smaller 12v runs/loads. Is that style breaker acceptable just fine in place of fuses? Or should I really fuse and then use dedicated cut-off switches?

For panels, just a note, that the 400W is solid that I WILL have them, because I know there are times when I need the higher loads. But if I'm going off for a weekend, 100-200W might be all I know for just a few days. Maybe even not that, if I'm going to be on the road for 2-3+ hours with the power from the tow vehicle.

I might actually start, for the longer trips where I stay in one place, to use the 2 large-ish old house panels (235W nameplate, I think still >200W actual production) for the time when I'm going out for longer periods. Physically a good bit larger than the 12v flexible, I think it'll work out fine for those trips. I think they're 56v Vcc, I've run them in parallel before (Renology 40a MPPT), already have that wiring with, I think, a decent length 4 awg run to give me flexibility in where I place them. So the higher panel voltage/amps capabilities would be useful now. Or with that planned use, should I just go up to the 150/45? I don't think I'll be going more than 400W or so, or if I do I might as well get a small 1500-2000W propane inverter generator at that point in time.

Yes the 712 will measure everything if wired correctly.

I fitted a similar fuse to the one you linked to, no issues so far, but the cheap ones seem to have a poor reputation for packing up. Especially when switched often. Better with proper isolator switch. Separate fuse is the way I' go if I did it again. Probably the midi or maxi, depending on what fits you layout best. Personally I'd go for slow blow fuses. But fast will be OK with a decent safety margin. Fuse blocks for the smaller 12V loads are good. Add switches for circuits you're going to turn on/off frequently, like lights. The rest can rely on the master isolator.

Watch panel voltage. If you're going to have them it connected in cold winters, the voltage rises, may blow your controller. Probably cheaper in the long run to go with bigger MPPT. Victron have an online calculator for selecting MPPTs.

Did I miss anything?

I think you've got them all of those questions. Although after looking at the charger/inverter page again, I see there's 2 MultiPlus variations (one with 500/800/1200/1600VA, one with 800/1200/1600/2000/3000/5000VA), the 500VA or one of the 800VA I think are physically small enough to fit reasonably well, and give just give me that nicely integrated and let me just change the shore power run right once. However, I seem to be having a hard time finding those lower VA versions for purchase anywhere, which is annoying.

For the fuses, after reading the first page of this thread on Class-T vs ANL fuses, I think I'll go with Class-T, at least right at the battery, and proper cut-off switches and either Class-T or ANL fuses for the rest. Other than the 12v lower amp circuits (like for the lights, although lights will have their own on/off beyond the fuse).

One last thought, is there any kind of battery thermal detect cut-off relay out there? Maybe I'm just being paranoid about the internal thermal cut-off, but if another $50-150 buys me that extra peace of mind, I'm kinda OK with it.

I'm thinking the Battleborn 100Ah 12v, even though it's a fair bit more than the SOK batteries I've seen mentioned/recommended on https://diysolarforum.com. This kind of investment, peace of mind can matter to me, and my budget does extend to investing in quality parts. Thus looking at Victron, although Renogy makes some solid stuff, not something I'd want in a more demanding environment like in a trailer. Plus Victron parts all integrate/talks to each other (except the Orion Tr....grrrr), and I'll trust the reputation more for a build like this.