Best way to check Battery Voltage?

Pete - digital voltage displays are very cheap these days and even the cheapest should be accurate enough. If you have a cigar lighter socket on the leisure battery circuit then something like this for under a fiver should do fine:

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I should add that I have never bought that specific product or used that seller - but they are all pretty much the same.

Otherwise it's quite easy to adapt something to plug into (for example) a Hella socket if you have one (the smaller type of 12V socket). Or, for less than a tenner you can get a voltmeter and twin USB supply which will plug into a Hella socket:

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BTW, I should point out - if you are not already aware - that 11 Volts is too low a threshold. If a battery sits at between 11 and 12V for more than a short period, the plates will start to sulphate and you will lose capacity, some of which may not come back when it is recharged.

[Edit - this thread suggests you have a Hella socket [You must be registered and logged in to see this link.] so the second of those products should work well unless you have something permanently plugged into it.]