Depends mostly on the chassis voltage.
If the chassis electrical system holds 14v+, then that's in the ballpark to bulk charge the house bank and an ACR is really all that's needed.
If the chassis voltage averages less than 14v, you'll want the B2B, which takes the chassis voltage and bumps it up to a higher voltage to do a decent job of charging the house battery.
A plus for the ACR is it will engage when the solar pushes the house bank to a high enough voltage and that will keep the chassis battery topped off (same when charging the house bank from shore power).
Another plus for the ACR is that the B2B will be current limited to some number, say 30a or 50a for a Sterling, so you'll never get more current than that to the house battery. The ACR (a 500a rated ACR), would allow whatever the alternator's current limit is. However, that's not really a big deal as most of the time the battery is going to be limiting the current, so you would rarely see the full alternator output anyway.
If your vehicle has a newfangled, half-assed "regenerative braking system", which keeps the chassis battery less than full so it can engage the alternator when you brake, to load the engine and bleed momentum into the battery... you'll need one of the new special B2B units that Sterling makes for that.