Watts are equal to the amps of current multiplied by the voltage. Watt hours is the amps times volts, times hours. So 1A at 120V is 120 watts and if used for 1 hour that is equal to .12 kilowatt hours. Most RVs use propane for heat but the furnace blower is 12V-dc current and that would be supplied by a converter that changes 120V-ac from shore power and makes 12V-dc to supply all of those loads. It is possible to operate most RV refrigerators on propane if they have an absorption refrigerator and if they set it to do so, as most of them operate in automatic most of the time and thus shift to 120V when available. They also may have a 120V heating element in their water heater or it might operate on propane, again an operator selection.
The 20 KWH per day is spread over a 24 hour day and if the loads were all uniform all day long, that would only mean it was using 6.95a constantly or the equivalent of that. That is probably a bit high, but not outrageous. If you figured 10 kwh per day that would be an average of only 3.47a spread over 24 hours. It is very difficult to estimate what the load is over 24 hours since it probably varies a lot. A factor that should be considered is what sort of outlet he is plugged into. If it is a typical house, 15a outlet then it would be difficult for them to consume 20 kwh per day but easy to reach 10 kwh.