Leisure batteries

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Hi. I need some advice about leisure batteries. I want as much battery power as possible as I may need to last up to a week without a hook up. I bought a relatively cheap 110AH battery but I'm not impressed with it. I have space for 2 of them but I'm sure they won't last a week. I don't mind spending some money to get what I want, but all of the higher AH batteries I can find are pretty big and wont fit in my battery compartment, unless they went in on their side. So my main question is, is it OK to use a gel battery on its side, ie. with the terminals on one side, not on top? I have heard that this is safe, and if so would be a good solution for me. Can anyone verify this for me, or recommend a good quality, high AH battery with a smaller footprint (preferably 355x175x220 max)?
Thanks