Ok consider measuring alternator output running the A/C and headlights at the same time.
In general I only recommend 20 amp DC-DC converter on a factory alternator as it can draw in 30+ amps to correct the voltage.
Yes there are circumstances where all will be fine at a higher level. If all...
80 amps from a 120 amp alternator for an added system seems a bit much. Many alternators do not have a current protection limit and can fail if they go over design maximum.
Best way to extract grid power with bad frequency is to get a charger that does not care much and charge the battery direct.
Same goes for voltage. Although not everything goes to 270. My Meanwell says 90 to 264v and 47 to 63 Hz.
At 15% delta on a fairly low draw seems a bit high. Start by checking the connections are clean, tight and balanced. Especially on the 9.2a slacker.
Next would be to measure voltage drop at each of the connections to find the issue.
Single 400 ah battery would work as long as the allowed discharge is 200+ amps.
Best to post a link to the battery before you buy.
Two in parallel is easy to balance. 2x the diy 280 with 120a+ BMS (each) would be my choice. Works well on my RV trailer.
Surprised to see the Samlex in with all the Victron.
My only real comment would be the battery might be a bit undersized. Inverter alone can easily draw 150 amps and while the diy 280 might be fine with a 200+ amp BMS the premade battery could have a lower limit. And while many cells are...
As long as the fuse holds, it would be OK to call it good. The real test will be when the battery is low. 125 amps off a 150 amp alternator could easily allow overload of the alternator. To know what is going on... a clamp-on DC ammeter would show amps getting to the house battery. Yes...