Not sure how the setup work on your trailer but I once saw a guy put a T piece in the pipe from the tanks and then put in a clear plastic pipe onto the T and upwards where it is visible. As the water level inside the tank drops the water inside the clear plastic pipe also drop. With some effort I suppose you can even add exact volume to the tank 5l at a time and then make corresponding marks on the plastic pipe giving you exact water level when the van is parked level. The only thing that I am not sure of is if the pipe had some sort of venting cap at the top or was just left open. If it is not able to 'breath' the air will just stay trapped in there and won't show the waterlevel.
I hope this all makes sense!? I have not build something similar but the idea seemed excellent and not very expensive to do yourself.
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2015 VW Transporter T5.1 132kw 4motion DSG - (Shrek) My Bush-Bus
2003 Safari Xplorer
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