The only holes should be the actual fill hole and overflow drain, both are typically at the same level. Now your tank may have additional options i.e holes for different mounting configurations but those should all have plugs.
I only boondock and fill my tank full. I put a longer piece of Pex hose on the overflow drain and installed a valve on that drain hose. After water runs out of the overflow I close the valve, this allows me to carry several additional gallons of water without it syphoning out during the drive.
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