While I am no battery expert or EE, I understand the basics of a Battery Management System (BMS), which I believe is important for a bank of house batteries. This is especially true when they are expensive lithium batteries on a 200k - 250k motorhome.
Since I am not sure which batteries you have in your boat or EC, I will pull these specs from the web:
A high quality 18650 cell should be charged and discharged around 1C for optimum life span (5,000 cycles +)
A high quality 18650 cell can be charged and discharged around 3C with a reduced lifespan (~3,000 cycles)
Tesla's Ludicrous Mode takes approximately 4C - 5C from the car's battery pack for short durations, but even this decreases battery life, as does completely charging or discharging your batteries. Tesla recommends stopping around 80% full and not going below 20% full. In their fancy $100K+ automobile, there is a very sophisticated BMS on board to handle all of these things.
If one were to take a Braille deep cycle lithium or make a custom pack using 18650 cells, I would suggest that you do some research on the parameters and capacities of that battery, whether or not a BMS is used. Let us say, for argument's sake, that you installed three of Braille's excellent i31D batteries (Group 31, Deep cycle) with 288 LAH. In theory, you would have 864 amp hours available, but HOW the battery is discharged will likely affect the results that you see.
How do you ensure that all three batteries are charged equally from your alternators? How do you ensure they are discharged evenly while using your heater, toaster, microwave, TV, etc.?
A deep cycle battery is often rated at 5H or 20H, for the 5 hour rate or 20 hour rate. Obviously, this means that you are discharging the battery over a 20 hour period, using relatively little power along the way. If you flip that around however, you would not want to spend 20 hours charging that battery back up. So, you must take both sides of the equation into consideration. If you had a pair of 150 amp alternators, running at peak power on a three hour drive, you would be able to charge your battery bank. This is basically a 1C rate, so you are not charging at high current, like a Tesla Super Charging Station does, for example. Any quality lithium cell should be able to take 1C, but check the specs for your batteries so you don't cook them.
Will the alternators only feed the house batteries? Will you have the ability to use to your house batteries to start your truck should your starting battery fail?
A popular Trojan deep cycle battery sees a loss of 40 amp hours if the battery is discharged over a five hour period (225Ah / 20h - 185Ah / 5h). So, I believe there is much more to answering your question that just connecting two additional alternators to a lithium battery pack, especially when lithium batteries and Earth Cruisers are quite pricey items for most people. It certainly seems doable if you have the space for three alternators and can spare the 5 - 6 HP it will take from your engine
