It will be much easier if you tell us the EXACT MAKE AND MODEL of water heater you purchased. Running a section of Romex wire and possibly running four 12v DC control wires may be a relatively simple task for someone with some mechanical skills.
But without knowing what you have purchased, there is no way to advise you of what to do or how.
'03 Ram 2500 CTD, 5.9HO six speed std cab long bed Leer top and 2008 Bigfoot 25B21RB.. Previous, Being repaired and will be sold, is a 2008 Thor/Dutchman Freedom Spirit 180. SOLD - 2007 Winnebago View 23H Motorhome.