I have a few battery questions.

All of my Cascadia sleepers have 8 batteries. No apu. We have dometic diesel bunk heaters and small camper AC units. The 4 batteries behind the bunk that run the heat and AC are deep cycle batteries.

Inverter also plugged into the deep cycle batteries.

 

I'm driving Cascadia with 8 batteries for running electric a/c when parked and it has 250-275 amp alternator. It also has Detroit Diesel Optimize Idle system. It supposed to start engine when batteries get low and recharge them when parked. It's a new truck and already had trouble with bad battery and lose connections. So now it seems to be working. It would be expensive upgrade to add 8 batteries and big alternator.

Plus getting wiring correct so you don't burn the truck down.

 

Wow!

Ok it would be a lot easier for you to get a generator, for a lot of reasons.

There is a lot of wrong info about the alternator, the batteries take the brunt of the draw with the alternator resupplying the battery reserves.

With a 3000 watt inverter, you will draw up to 15% in addition to the 3000'watts, some have overhead and that consumes power. Overall with a 250 to 275 amp draw, the batteries will go flat fast.

 

Do not confuse AC amps with DC amps,
A 1500 watt device like a coffee pot will draw 12.5 amps at 120 volts AC.
To generate that 1500 watts AC thru a DC inverter would equal 125 amps at the battery cables. Run 3000 watts worth of power and it will draw 250 amps.
Watts= volts x amps
Amps = volts / watts
always

Right, and those inverters are drawing the surplus DC amps from the truck batteries for the duration of the electrical load. Watch your trucks ammeter peg when you turn on a 1500w device. Why do you think 1/0-2/0 cables and 200+ amp fuses are required with a big inverter? Because of the DC amp draw.

 

Should be a "Pad mount" generic style usually.

Leece Neville has shelf 320A Alternators.

I personally think the Delco Brushless is the best you can buy. Look at the 40SI 300A, 320A or maybe the BEAST the 55si 430A unit.

They also have voltage sense capability to charge your battery Bank much better than a standard alternator if it's hooked up.

Dometic used to sell a very nice aluminum battery box that is adjustable and goes between the frame rails. Holds (4) group 31 Batteries...

 

I have a 2500 watt pure sine wave inverter. They are not 100% efficient. Most are about 85% some more some less.
A 2500 watt load at 120 volts ac would equate to roughly 20.833 amps, but to supply the inverter from a dc load would require about 10 times more amps. If you divide the voltage by ten you increase the current by ten to maintain the same load in Watts. Roughly 208.33 amps dc.
Now you divide 208.33 amps by the efficiency of the inverter 85 % you get a more accurate 245.09 amps dc draw on your batteries.
And that’s only for one 2500 watt inverter, you want to use two 3,000’s you’d be closer to a 600 amp draw. The reserve capacity on a lot of batteries is around 180 minutes, but that is only based on a 25 amp load.
So you would need around 24 batteries and that would only get you three hours of use.
In practice I only run a microwave and a refrigerator. The fridge needs the extra over head to start the compressor, but once running only draws under 200 Watts and that’s only a few minutes per hour. My microwave draws 700 Watts , but I only ever use it with the truck engine running. The inverter indicates close to 100 amp draw when the microwave is in use.

 

I think a generator will probably weigh less and last longer than the setup you're considering. Gasoline will cost less but be more hassle than a diesel that you can plumb straight to your tank.

 

Have you thought about about going with deep cycle batteries and putting in a start module to start the truck. That way if you will always have Power to get the tuck started and the deep cycle will work much better with the kind of Power your looking to run.

 

I have 4 deep cycle that power an inverter, diesel heater, and an AC unit.