Nuevo ES leaking fresh water tank

We seem to have a leak from the fresh water tank sited under our 2007 Nuevo ES.
Attempts at tightening the red cover haven't solved it.
Is it usual to drain the water from this sump? We realise our instruction manual refers to a tank under the passenger seat, so no reference to a sump drain!
If we can undo it, would the addition of pfte tape help?
But we think the leak might be coming from where the screw thread is welded/bonded to the tank....if so what can be done?
Any suggestions appreciated :)

My Nuevo ES 97 is on the earlier Boxer x244 shape so may be different to yours if yours is on the later x250 Boxer. To drain the fresh tank on mine, I turn the tap in the sill on the same side as the filler. The hot water tank drain is the dump valve under the offside seat. I've never undone the large sump plug.

Yes, 'praps its different!
We've drained in the same way, but when we discovered the fresh water was leaking and had a look at the tank under the van, we found it had a large red sump drain. As I say, not as described in the instructions!
Can anyone else help please?

Yes, that sounds exactly the problem! Many thanks Cyclo 
Now we'll attempt to open the red cover, having just tightened it!

I had the same problem on my 2010 Broadway. The leak was from the flange that the big red cap screws onto. It is held onto the tank by about 8 - 9 self tapping screws and a sealant between the surfaces. It is possible to fix it yourself, but working under the van isn't to everybody's taste. Mark the position of the flange so that it goes back in the correct place. Take the cap off and unscrew the screws from around the flange and when they are all off it is possible to ease the flange off. It then needs to be carefully cleaned up and then replaced with a good quality sealer. I used CT-1 following a recommendation on this site. Its been fixed now for about 3 years and I haven't had any further trouble. I did think about asking Marquis to fix it for me, but they wanted a large amount of money to take the tank out and investigate the problem, without knowing how much they would charge me to fix it, so I decided to climb under the van myself. 

My freshwater tank has a large BLACK screw on inspection cover on the base.  I have not yet investigated it as I do not want to start a leak!
If only the waste tank had such a screw cap! 

I think China explorer has the answer....many thanks for the detailed explanation! 
We'll send our son under the van 

I had the same problem on my 2007 Nuevo. The inspection hatch cover on the bottom of the tank unscrews and there is an "O" ring that makes the seal when you screw it back on. In my case the leak was caused by the "O" ring becoming hard and distorted. The tank cover assembly is a Henderson part number TCL4 However they normally only sell it as a complete assembly with cover & flange. In my case they very kindly offered to make me up an "O" ring which took 5 days & cost about £5.

[You must be registered and logged in to see this link.]    My 2009 model has a BLACK screw-on inspection cover and the flange thing it screws into is NOT held in place with screws.  I managed to undo the cover and the "O" ring is thick and un-perished so I think it is the junction of the tank with the flange that is the problem.   Crawled underneath today and applied CT-1 sealant to this junction and will give a second coat tomorrow before re-filling.  Only if this fails will I seal the screw in cover!

Good evening and welcome to this most friendly and informative forum.