Hard question to answer without knowing how they've been maintained. If they've been well maintained and hold a charge okay you probably don't have to replace them. I know that tires need to be replaced based on age, but I don't know that the same rule applies to batteries.
However, that said, I replaced my motorcycle battery after 4 years even though it was holding a charge and doing okay -- I just didn't want to be caught out somewhere and not have the bike start. The coach batteries in our RV may not be as critical as far as leaving you stranded since I could always jump from my TV.
I think I'd do voltage tests and if they are holding okay I wouldn't replace them until they started showing signs of age/trouble.
Then again, if money isn't that tight, it might be worth your piece of mind to just replace them if you're starting to worry about them.
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Dave in AZ
2008 Montana 3400RL
2008 Dodge Ram 3500 Diesel Dually
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