Too hot for some

YOU don't say how many kilometres your car has done. If it's done plenty it might simply be heavy wear that's causing a loss of oil pressure. Because you say it only happens on hot days, or when towing your caravan, it probably won't show up at other times, such as when you have it checked. Start by making sure all is well with the engine, spark, fuel, etc, and thoroughly check the cooling system to make sure all is well there. Change the coolant and use the recommended coolant, check the fan belt, water pump, and thermostat. If everything checks out you might think about an oil cooler.