Have you verified there is hot water in the heater and it's just not coming out the taps?
Have you turned the water heater's electric switch on (if so equipped)? I know this sounds silly, but you are plugged into a form of shore power for the electric option to work (it doesn't off batteries)? If you are plugged into a form of shore power, are you using any kind of adapter (ie to plug a 30 amp RV into a 15/20 amp house outlet)?
This link may also help if you have a dual powered Suburban water heater. There's a possible hidden switch that a lot of people miss.: