[BiggarView]

You say you will be boondocking a lot. With a residential fridge on board you need 140 AH @ 12v every day just to run the fridge. Add in Tv's and laptop(s), other electronic devices, hair dryer, microwave, crock pot, toaster, coffee maker and so on... you could easily push 300AH a day. Four 6v 400AH AGMs (giving you 400ah @ 12vdc usable) would be a bare minimum and you need to recharge from your genny daily for at least 3 and maybe 5 hours a day just to keep up. That will suck down propane at an expensive rate. Also, you risk premature death of those batteries if you drain them down past 50% every day. LFPs do not have that issue. If you only boondock in moderate climate that will help with LFP because they do not like being charged in very cold weather or extremely hot weather. AGMs less of an issue. With LFP, they take a full charge quickly and all the way to nearly topped off so you can cut genny time in half or more... the extra cost up front can now be measured by significant savings in propane costs. Plus you are off the noisy genny more hours every day, if you don't enjoy that benefit... your neighbors certainly will. With a decently sized inverter/charger or a converter you can put back 100Amps every hour until you are near topped off with LFP, not so with AGMs which is why AGMs take so much longer to charge up. With 500+ watts of solar adding into the mix you might get 100-200amps a day in good weather further reducing genny time. If all of this allows you to boondock even more then you can figure in savings from less time in per night campground fees. But for now, your only real savings will come from less run time on the genny and less propane consumption. To see the real benefit of the larger battery bank you need will need spare battery capacity with AGMs to prolong their life, not so critical with LFP. You can get by with nearly half the battery bank with LFP than AGM.

So with 4 6v 400AH AGMs weighing ~500 lbs vs the weight of LFPs at ~115 lbs there is a huge weight savings. True the AGMs will cost about half the LPF but they will not last as long, will not charge as fast and they take up more real estate in volume than LFP. But it is the weight penalty that for us was the final nail in the AGM coffin.

If you are only a recreational RV user then the weight issue may be a non-issue. Full timers with moderately capable rigs like yours and mine 3813MS with ~3100 lbs of CCC those few extra hundred lbs could be the deal breaker... it depends. If AGMs work for your situation, by all means get them. We are going LFP because they work for us. YMMV

One final thought on the AGMs. You are looking at four 6V 400AH(presumably) and placing them in the existing battery compartment on the front left side. The 3813 MS already has a potential weight issue on the left side... 2 long slides, the fridge and the pantry are both on the left side slide. Mix in the fact that the rig has only 7k axles. that is 3500 lbs per tire position. The fact of the matter is there is real potential to overload either the tires or the axles and bearings on that side of the rig if you load the trailer to it's GVWR. Many load way over their GVWR. It does sneak up on you especially when full time RVing, or so the anecdotal evidence suggests. You are increasing the likelihood of a driver side tire failure with all the potential damage that entails... food for thought. An overloaded tire will most definitely ruin you day at an unexpected moment. Factory "nevr lube" wheel bearings tend to fail catastrophically without warning and that will destroy an axle and tire almost instantly. Cha ching. Or you can avoid that by regular bearing replacement (also cha ching) These are potential side effects of piling more weight into your rig. Sorry if I drove into the weeds with these other considerations but your battery choice also affects these things. Weight is your enemy. You choose your poison.