I've seen ones that are clear or translucent so that you can see the level inside. Boat owners like them because steel rusts and aluminum corrodes in a salt air environment.
If they have a DOT certification and date stamp, they should be fillable, but some may afraid of what they don't know.
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John & Nancy
Jay Feather 25Z 2007
Ford F-150 XLT 5.4 Triton 2008
I don't know the exact company name but I think there was a recall on one of those brands. I saw the actual recall notice (so I am not trying to spread rumors) but just can't remember the name.
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Skip
2012 Eagle Super Lite HT 26.5RKS
2005 GMC 2500 SLT HD D/A
Location: Luray, VA (central Shenandoah Valley of VA)
Posts: 1,430
I wonder if these are susceptible to puncture ?
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2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, Teddy (Jack Russell terrier), and Honey (Beagle)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.