AGM batteries and engine heat? Or heat tape?

If the four batteries are sandwiched right you should only need 1 or 2.

Thanks for the blanket tip -- I didn't know they existed! I use low wattage home or pet heating pads on my ice machine.

The blanket that just wraps the center of the battery like a cummerbund seems like it would be good and not insulate when unwanted. And 80 or so watts should warm up quickly. $$ for 4, though...

https://www.amazon.com/Kats-22200-Watt-Battery-Thermal/dp/B000I8XD9E

Are you going to sub freezing temps for long stays?
Most RVs, have the main battery compartment is exposed to outside air
Like being under your van, but not as close to the street
They suffer no cold weather problems and don't need to be warmed up

Are you getting AGM chemistry confused with lithium chemistry, which can not recharged when cold?

No I would not put them near the exhaust or close to the hot street, how high is the van frame, how high/far Above the ground will the bottom of the batteries be?
They should be at least 18" above the street, the higher the better
Unless your going North in the winter, cold is not a real concern, but summer heat is

And remember, batteries survive being inside the engine compartment, with 200 degree engine temps hot radiator air flow, etc..
So even heat is a moderate concern, unless exposed to hot exhaust pipes

I would not get heat tape but rather a regular battery warmer.

https://www.amazon.com/Kats 80 watt

This item is for cold use only so I would add one of the theremocube or other thermostat. I would let one heat both batteries.

I've been using AGM for about 20 years in my ice machine. My experience is that they are good (albeit a bit weaker) down to about 40 degrees, but once they get into the 30s they are nearly useless.

When I move camp to a place with warmer nights, it takes maybe 48 hours for my 45aH ice machine batteries to warm up, just from ambient temps. I'm concerned it may be a very long time for the bigger ones!

They have insulated battery heating blankets, they're 12v I believe.

Protect from the exhaust the same way, put some fiberglass between them and a hot exhaust.

AGM don't stop working when cold they're just less efficient. They tolerate temperature extremes better than a flooded battery.

Installing in a tight insulated place, you'll definitely want a temperature compensated charger.