A little more info on motor and RV would help. Do you get a wait to start, if so this can take a bit of time as it is heating a grid(an electric heater for intake air) to give warmer air to start. If this grid has failed for any number of reasons it ill be hard startIf not you may need to use the block heater for a hour or two to get the warming for start. Also are we talk diesel coach or Gen? If Gen it will have glow plugs that warm for start, there are fuses under the Start switch on the Gen that maybe burnt or the glow plugs may be bad.
Been there and had that....Diesel in cold weather will gel...if there is a napa ,walmart or ? near get you a bottle of 911 diesel treatment...Suggest you keep a bottle handy if you are going to vixit cold country The plastic bottle is red... Chuck G. Wait approx. 20 to 30 minutes for treatment to dissolve