Hello! My first post here as I have purchased a 2020 sprinter camper van conversion that came with a Victron equipped solar setup. My system is 3 solar panels (unsure of the output) running into a Victron MPPT, then down to a Victron Multiplus 2 compact. Storage is via 4 - 100ah Battle Born batteries. My particular problem is on the AC 120V side. Van is quipped with 2 wall plugs each having 2 plug ins. The previous owner has purchased a Bosch 2.5 gal hot water heater and plugging it into one of the 120V plugs. When turning it on, it will run for about 3-5 seconds before the Inverter trips “alarm” and cuts off. I have verified that the water heater does run fine on shore power at 119V and heats up the water. WH is pulling 11.7amps according to my clamp meter. The outlet is on a 20a breaker which does not trip. I have the battery monitor and it switches to “low battery” when the inverter cuts off, but i believe thats just because the inverter completely shuts down. The battery on the screen shows full and charging only last 1 minute before wattage comes down on shore power, so batteries are full. The battery monitor logs a draw of 1660w max when the water heater kicks on and is first running. This is where i am stumped. The water heater runs 119V, 11.7amps, and heats the water fine when running on shore power plugged in. The plug in which runs the water heater is showing 119.7V when tested with nothing attached or drawing from it. Wires on the 120V AC from the inverter to the breaker and then to the plug are all tight and correct. So what is happening to cause an alarm on the inverter? If i calculated correctly, my battery supply should last around 2-2.25hours non stop on that water heater with no other electrical drains. Also, its a multiplus II 12V / 2000VA / 50A inverter. Any thoughts on this? thanks! Josh
Based on 11.7A at 119V that is a 1400W water heater. Depending on your Multiplus temperature (how well is it cooled / ventilated that load is higher than the continuous rating of the inverter. See datasheet snippet, hopefully I have the correct model but most will show similar derating. Additionally, at start up the element is cooler and it will have a lower resistance and draw higher power until it gets hotter. The other issue to check is that the wiring from the batteries to the Multiplus is large enough and has no loose connections and all fuses and breakers are making good contact.
Thank you! Yes, it appears this is my issue. I was unaware of the continuous rating of the multiplus compact and i guess the original owner of this van was too. Not sure how he ever made it work, but it does not now. I have searched for a lower watt heater element for it, but no luck so far. I could get away with an 800W or even 1000W heating element, but everything i find is a 1440W for the Bosch ES2.5 water heater 
I guess i will be taking a few more cold showers and either switching to a 3000W inverter or going with a diesel heater/water heater setup in place of what came in this van. Thanks for the help!
Try to turn down the output voltage of the MP to 100v. Then the heater should get 1000w, the MP can breathe again.