Standard 30 amp RV service plug end looks like this:
Replacement Cable assembly looks like:
If need be, you can buy a 30ft extension cable from Walmart then cut off the female end and wire it in (Walmart.com: Road Power 30-Amp RV Extension Cord, 30': Computers
). If your rig has a detachable cable then you don't have to modify it.
If you have the standard cable with just an incorrect plug then you can just buy the plug end at Lowes:
You then can purchase a 30 amp to 15 amp adaptor simular to this:
Even Walmart carries a few different styles of 30 to 15 amp adaptors in their RV display in Automotive section (Walmart.com: Road Power 30/15-Amp RV Power Adapter Cord: Computers