From the OP: Yes climbing under the van is a bit difficult now, but I dont have any more mobility issues than you would expect for my age (72). But I have looked underneath and neither these tubes nor the scuttle drain tubes can be seen.
However since neither I nor anyone else has any idea what the tubes are for, I am reluctant to pour water into them to see what happens. For instance, what if they are to top up some fluid somewhere or maybe are vent pipes from something that needs to be kept dry?
No offence intended on my part either, but if you are not a X290 owner then please skip to the next post. But if you are a X290 owner then perhaps you can bring more experience and intelligence than I (not difficult) to examining them - just under the scuttle. See what you think.