There are many, many good posts and resources considering all types of batteries on IRV2 - you can read for hours. One approach may be to get some accurate measurements of the space and then research the various types of batteries and their dimensions (keeping in mind that you need access to the top of 'unsealed' batteries). Lithium batteries are capable of being installed on their sides or ends if needed - a helpful benefit in tight spaces. Alternatively - lithium is best installed in a heated/cooled space for optimum function and life. Given lithium batteries construct and properties folks with battery compartment space constraints locate batteries in some other interior space. Best wishes ... let us know what you decide - we all learn ...
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Steve
2015 Itasca Ellipse QD | 2014 Jeep Grand Cherokee
Go with either 6v golf cart batteries or agm batteries. Personally stay away from lith-ion for now. Besides you may have to change your inverter charger to support them also.
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Retired Navy Submariner
2014 Itasca Sunstar 35F; 5 Star tuned; 2014 Jeep Cherokee TrailHawk
If the current batteries are over a year old it is normally recommended that you do not add new batteries into the bank but install all new.
Unless your previous coach had two batteries and you know how that fits your usage I would wait before adding/replacing until you see haw the current setup works for you.
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Paul, Kathy, and Tux the Mini Schnauzer
2014 Tiffin Phaeton 42 LH, 2013 Honda CRV
"When the time comes to look back, make sure you'll like what you see"