Yeah, that's the big downside to AIO's. Because they have so many things going on inside and each internal piece is designed to be cheap rather than efficient, each piece uses a bit of standby current. A 3Kw AIO inverter is usually about 60-ish watts per hour to exist and be running. The 6048 version is really just two of their 3048 units (3000w, 48v = 3048 for naming convention) slapped together inside the same box. So, if each unit is pulling 60w an hour to exist, and you've got 2 units, that's 120w per hour just to feed the inverter. In a 24 hour day that's 120 * 24 = 2880 watt hours of battery to exist.
So there are three ways of dealing with this problem:
1: Use a smaller AIO that doesn't draw as much power and limit your usage, like just using the single 3048 for 60w/hr
2: Spend a fortune on Tier-1 Super efficient components that you assemble yourself to save standby power and skip the AIO
3: Spend more money on another battery
The EG4's are about $1500 right now and I think are the best bang for the buck rackmount batteries out there. That's 5Kwh of battery of which you'll lose about half to inverter standby, but you'd have to compare the $1500 for a second battery VS the cost of all the Tier-1 parts that might save you half your standby draw or less.
So here's an idea. Since you're looking at the MPP units anyways, start out with a 3048 alone and a single rackmount battery or two. Have your electrician friend install a 30a AND a 50a plug into the side of the container and either have him do the jumper thing or just get the off-the-shelf adapter. Then, in the future get another 3048 and the communications cable and install it right next to the existing one to give you that 50a 240v socket. Move the plug and be done with it. If you find you need more than the 30a per leg that gives you, you can always add more 3048's in parallel to increase your power availability, but I doubt you're going to need to if you're a bit wise about your power usage.
Or go for the gold and get a second battery and a 6048, park your container so you have a long side facing south, build a frame on top of the container and start loading it up with all the panels you can use up there. With a 40' container and careful panel choice you might be able to squeeze 4Kw worth up on the roof. It's a lot up front BUT that's super simple and should last many, many years of feeding a camper when you rent it out to the in-laws.
As to feeding the well pump, that's a whole 'nother can of worms. If it's shallow enough you might be able to feed it from the 6048 you've already got going OR you'll have to set up another smaller system designed to just feed the well pump. It can be done but it's not super cheap because most well pumps have HUGE startup draws and your Low Frequency inverters are pricy! I've looked at building a solar backup for my well pump and I think I can make something that will barely work for about $2000 as long as I don't need to run it for more than an hour a day or so. But then again, my pump is 300ft deep.
