Never seen the need, it's already "outside" the heater. The water just runs out the bottom of the access panel, no big deal. They really shouldn't vent all that often anyway.
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"I can fix it, and if I can't fix it, I can fix it so no one can fix it!" Ed & Wendy 2009 Georgetown 378TS | 1998 Jeep Wrangler | 1998 Skeeter ZX202C
Nights camped in 2009: 53 | Nights camped in 2010: 55 www.nwjeepn.com
Never have, I would think if you were to do it the extension would have to be metal of some sort as the heat build up would melt a rubber type hose. Also just another place for mud dobber to live.