There are many others styles. My 276 has a round cover over a gravity fill port with an overflow vent along side .... both covered by one round flip style cap.
The water inlet for 'city water hookup' has a screw on cap.
You don't want bugs, spiders, dirt, etc. getting into your fresh water.
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2012 Fun Finder XT 276 Toy Hauler & Harley Davidson Ultra Classic with Condor wheel chock
2014 Dodge Ram 2500 AT 4X4 Crew Cab 6.4 L Hemi, 373 gears, tow package, Rambox option, Revolver X2 tonneau cover, Equal-I-zer anti-sway hitch.