Yvonne, on our last van it had heating pipe running under the van for the loo, and it never really got hot air going through to the loo it was always just worm in winter. Our new van has it under the bed to the loo and this week whilst away in the cold but lovely weather the loo was the wormest place in the van.
What we used to do was to leave the doo ajar to let the air circulate.
As for lagging the pipe it would need to be waterproof as it is under the van. Id say leave it and see how you get on, we really never felt it was a problem
Kevin H