What fridge.......Dometic/Norcold and which one of their models????????
Where are you measuring that temp at.........freezer or food section??????
If freezer....temp wasn't good to begin with (28*F) and if food section 28*F was too cold and 40*F is acceptable??????
How are you measuring those temps????
Electric or propane.......although they have different btu 'ratings'......the actual heat used by each for the cooling process is relatively the same.
So if both are function correctly..either will provide same cooling effect.
Things you can check are cleanness of burner area, electric circuit continuity, obstructed ventilation/fallen down insulation, any yellowish powder residue in outside compartment
But Brand/Model gets specific info.