There is no such thing as a sealed graphite battery...except perhaps in a marketing department. Sounds like you are looking for an AGM (absorbed glass mat) sealed battery and you need one that is a TRUE deep cycle. Excide does not make one....despite labling their megacycle marine as one.
If you have a modern 3 stage charger/converter in your Berk.... you will not cook a conventional deep cycle AGM. But they do require SOME differences in care while relieving you of standard water duties. In Particular...they should always be recharged fully, NEVER left to cook on the shore power for weeks or months unattended, vented properly to prevent overheating in hot climates and never equalized.
I would suggest AGM's from Trojan or Deka Marine (DC model #'s) and simply buy the biggest ones that fit in the space you have available. If you can fit FOUR batteries I would suggest the Trojan Golf Cart 6V AGM's wired in series/parallel as being a great choice.
EDIT...Just an additional note. There are some MUCH cheaper sealed batteries which are often used in car STARTING applications. In reality these are WATER/LEAD/ACID based conventional batteries with a sealed water reservoir. They're fine for cars but AWFUL for RV's and WILL get cooked as the water boils off in deep cycle use and cannot be replaced.
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