"Because the truck only gets used for trips and may not come out of the garage until March, I'm using the manual bypass switch to let the Iota charger (and IQ4) keep the truck batteries charged. Other than the 3 batteries seeking slightly different voltages, anybody see a problem with that?"
I've had the same question and, in fact, my truck is currently parked in my shop with the camper's Iota charger maintaining the camper and truck batteries. I reasoned that it is OK, because the Iota is basing its charge rate off the SOC of the camper's battery. The truck's battery always has some draw down from the truck's electronics, so there is always a small draw out of the camper's battery to the truck's battery. I notice on the Trimetric meter that there is a small charge (amps) going into the camper's battery, even when it is at 100%. I assume it is filling the drain from the truck's battery (and electronics).
All of the batteries are AGM, but they're not the same brand so the charging specs would not be exactly the same. But I think the float charges would be close enough to maintain all of them off the Iota. I'm not sure if my reasoning is correct, but this was my thought process.