Welcome Iain. Your tank should not leak. If 3/4 full the level will be below the bottom of the exterior fill entry. You have a breather hole in the internal screw top, but water could only come out of this if you over fill the tank or tow the van when water is in the rank. You should not tow the van with water in the rank.

Colin

Iain, water will come out of the breather hole in the cap if you go to the very top. We are now on our 4th Eriba with a large tank and have what we think. Is a foolproof method.
We have 5L Truma electric hot water heater as well and use the following method. Before leaving home we put 2 full watering cans in from outside. We open the taps, prime the system and then close the taps. Next we pull the bung out of the bottom of the tank. When we get to site 3 cans go into tank and our bucket goes under the waste outlet. Every time we empty the bucket another can of water goes into the tank. After a few days we usually have to put an extra can in.
This has worked perfectly for us on all but one occasion. I was wondering why the lovely Mrs eribaMotters had managed to empty 6 watering cans, that is 50/60L into the tank that should hold 35/40L. Answer, I had not put the top back onto the tank. Result, a very wet floor that fortunately dried out in the 35+c.

Colin