Walmart may offer only a 15 day guarantee because they may not wish to get too deeply involved with all the aspects of warranty returns/repairs/replacements etc. If it fails within the 15 days they most probably would simply exchange it for another one and send the faulty unit back to the manufacturer under some arrangement that they have with the manufacturer. It gets the early failure products back to the manufacturer quickly, so that they can evaluate if there is a major problem with the product that they have to rectify.
To my mind this makes the retailer 'look good' to the customer as they will replace the unit usually no questions asked (as long as it doesn't look like it has been damaged by the customer). The customer is mollified and thinks that the store is great and that it must have been a 'one off'' failure of the product so the manufacturer in the customer's eyes also maintains their reputation as the customer didn't have to deal with the manufacturer so early into owning the product and the replacement product will be OK as nowadays electronic devices are usually very reliable. Do not know about RCA but some company's customer service is abysmal. Call me cynical but ask yourself how would you have felt towards both Walmart and RCA if it had failed within the first 15 days and you returned it and they replaced it no questions asked, besides seeing that it was faulty and having to show the purchase receipt of course and the replacement is still working OK?
The manufacturer offers a 12 month guarantee so consult the warranty information that came with the tablet as to what you have to do to make a claim for a warranty repair or replacement with the manufacturer.