Rich, you can measure the battery cases of your existing batteries to determine the amp-hour rating; most battery suppliers will have a chart indicating the amperage rating based on the physical size of the case. But it’s always been my advice to carry as many batteries as the space and your wallet will allow. You can never have too much battery capacity. Many motorhomes use Group 27 (or larger) batteries for their auxiliary DC power. Some go with 6-volt batteries wired in series. Be sure to have your existing batteries tested properly before condemning them; though at five years, they probably have indeed expired their useful lives.
Put as many Group 27s in there as can fit. You’ll want the highest amperage storage capacity as possible. As an example, two Group 27, 12-volt batteries wired in parallel will equal about 210 amps of storage capacity. Two Trojan T-105, 6-volt batteries wired in series will yield about 225 amps of storage. If you can fit three or more Group 27 12-volt batteries, then go for that. When using 6-volt batteries they must be added in pairs. But do the math....four Group 27s in parallel will provide 420 amps or so, but four T-105s will provide 450 amps total, and with a slightly smaller footprint.
More is usually better! You may also want to consider an upgrade to AGM batteries if you plan on extended dry camping excursions. Lots of options to consider!