100% correct about using electric in freezers. You should not contemplate using LPG trucks under any circumstances. Four basic reasons:
First, is related to energy efficiency, the air in the frreezer is not changed, (other than when the doors are open), just recirculated. This greatly reduces the energy input to chilling "new" air at ambient temperature.
Second, due to the first even an LPG engined truck working as intended will emit a small amount of CO, accepting this, the level of CO will build up over days and months to potentially high and dangerous levels.
Third, the truck emits heat which hurts the thermal economy of the freezer.
Fourth, the truck fuel system / emission control system may not be able to effectively control the air fuel ratio and hence CO emission level due to the temperature in the freezer, (dependent on how cold it is).

Hope that this is of help, regrettably all the forgoing knowledge is the result of personal involvement in this issue! truck he retom