Washing Underneath

Has anyone experience of using one of these:

http://www.amazon.co.uk/K%C3%A4rcher-26427390-Karcher-Chassis-Cleaner/dp/B006ZLET9Q

I am referring to the underbody of the MH     

PB

No, I  haven't, & I think I'd be a bit wary since there are many openings under our vans & i'm not too sure as to the water-tightness of the floor, sure, it'll not get very wet under 'normal' driving as the spray is going towards the rear, but to actually SPRAY uphill?? Hmm, 
There's a few bored/drills openings that may NOT have been treated & wet in the ply/sandwich could induce local swelling etc. & you might even get wet inside somewhere.
It obviously can handle normal road, wet conditions, but a total 'upward-monsoon' might not be such a good idea. 
I was amazed last summer when I had a look under to find the entire centre of the floor to be dry, only the sides were wet (from the wheels) & that was after a long wet run home.