I have a 7" Android RCA VoyagerII tablet. The instructions say I must use the very short cord charger that came with it and not even try anything else. Is this true or can I use a regular long corded phone wall charger that matches the connector?
Why the OEM is advising you is to use the charging cable that was supplied since
Shorter cables are normally thicker and offer less resistance
Impedance of the cable is matched to the charger
On the other hand, longer cables are generally thinner and offer more resistance for charging current and takes longer to charge, besides the impedance mis-match at phone and charger ends for charging to happen as rated.
Do stick with OEM recommendations, unless you find a long cable that matches same performance as cable supplied.
Edit: For a detailed explanation which answers "why short cables are better and how to select a USB cable", see my answer How to choose micro USB cable for my device?. I believe that would fully equip you in making a right choice