AGM batteries?

I've had this same system in 2 trucks over the last 7 years, I'm very familiar with the good and bad of it. The heater parts are expensive with all of these heaters. Batteries take a beating and eventually fail. My 2500 watt inverter is pounding my batteries as well, with a microwave, and coffee pot (always on during the day). I run the truck any time either of these are at full load. I also have a full time wabesto fridge which is 7yrs old and still wonderful.
My inverter runs 24/7 and sounds off with an ear splitting alarm at 11.4 volts, by then its getting chilly in the truck any way.
With all 6 batteries after 8 hrs of heater the volts are in the high 11's. Using 2 isolated from the starter batteries I get any where from 5 to 7 hours until the alarm goes off at 11.4 volts. If all the batteries are in good order, no weak or dead cells, I've never had any problem starting in the morning. But it only takes 1 cell to go weak to cause failure.
In the OP my interest was in AGM batteries to improve my AH using the 2 isolated batteries. But the esm module seems like might be a good solution. The price holds me back, since in many ways it just looks like 1 battery, not only to me but to my wife too.

 

The x45 draws 7.5 amps at full power, but that doesn't include the standard sleeper blower motor which I use. When I initially installed it in my first truck I followed the installation instructions, and followed up the same way on this truck. I believe there are low amp draw blower motors that will mount directly as an OEM replacement. That might be a critical AH component not be addressed as a battery saver.

 

I want to revisit this topic with a slight twist from the op.
I've decided I'd like to swap out my 2 standard flooded batteries, used for heating, for 3 Duacell group 31 AGM batteries. Leaveing the 4 standard flooded starter batteries unchanged. Can I charge both sets of batteries with my alternator even though they are at different levels of charge, different design? Do I need added devices to regulate them separately?

 

I went ahead and bought 2 Duracell AGM batteries today, it was what they had, and ordered a third. I'm going to install them inside under my bunk so they stay warm, seems logical uncleal13. I'll have plenty of room for the 3rd or more. I plan to leave the solenoid in line to isolate the hotel bank while the truck is off. I also ordered a 2 battery bank 2x10 amp onboard Ginius smart charger for when the truck is parked. Apparently AGM batteries need to be fully charged in stages. I'm anxious to try these out, they sure are beefy.

I'd never considered 2 heaters for separate purposes, it seems logical. Frankly I've never used my Proheat as a preheater, I just use it when its needed. I've no idea how well that would work. I plug in my block heater & tank heater at home.

Will the Maxwell esm start a cold motor by itself?

 

To clarify my question.
If I went with an air heater, allowing the motor to cool over 8 to 10 hours would the Maxwell esm be enough to start the motor? I assume it has its limits related to cold weather, any one care to share experiences?