Need some guidance

[Rawze]

first of all,.... a 1500Watt load on an inverter is actually a 1806 watts draw on the DC side of it. This is because an inverter under full load is usually about 83% efficient under real world testing despite the inverters ratings.

if your voltage is dropping that far down, due to inadequate wiring size from alternator, or alternator or batteries getting weak.. that is roughly 180+ amps. -- The truck itself needs another 85 - 96 amps to run itself, so that is a total of roughly 276Amps load. -- Your alternator is WAAY TOO SMALL!. The rule of thumb on a continuous duyt load on alternators is a factor of 1.4 ... -- and also, the likely-ness of the factory alternator wiring from the alternator to the batteries not being big enough is also high. Unless your truck was specially ordered with high current alternator and bigger wiring harness... your going to need to replace the wiring from alternator to batteries to make it right too.

* A 325-Amp alternator. It is a standard size and is big enough to hold the continuous loads of 2100-2800 watts. I got one for my own truck at fleetpride a few years back. A standard pad-mount 315 or 325 leese-neville alternator works just fine.

* A high-torque (or toothed) belt for the engine. ..
   
I use this belt. It is difficult to find the exact one with the extra gripping-power, most places sell the cheaper non-gripped version of the belt. Here is the correct one...
https://www.summitracing.com/ga/parts/ce...2/overview
or
https://www.walmart.com/ip/Goodyear-Belt.../473358550


* Al least 2/0-awg but 3/0-awg if you want less than 3% voltage drop. All the way from the alternator to the batteries. And might as well use same gauge wire to the inverter too while your at it.

* (for your 1500W inverter) 180-amp fuses at the batteries for the inverter. DO NOT put in bigger fuses than your system can supply. Let the fuse blow to remind you that you are using too much power on the inverter than your truck alternator can handle ... keep spares handy.

* An inverter that has a continuous rating of 1500+watts and peak surge of roughly 2,100-w or more. You say your is 3k... this should be big enough... but you should not be using much more than about 1500-1600 off it total ... unless you want to go even bigger with the alternator and wiring.

* The better quality and lower amperage 700-cc amp AGM-GLASS-MATT batteries. I would install at least 6 (instead of 4) of them for the loads you describe and truck. Battery boxes are avail for the passenger side of truck for optional electric APU's. Might as well install 8 (2 sets of 4) if your going to install the second battery box. -- Higher than about 750 or so CC amperage batteries will only puke out on you prematurely as the plates in the 900+ and especially those 1100+ batteries are way too thin for to handle the high loads your placing on them.

this is the reality of what you are trying to achieve according to your post above.... Stop under-thinking it, you have underestimated your actual loads ... You want to do it right,... that is how it should be done.