Glenn, it depends on what the nose weight [the weigh the van drops onto the tow bar] of the caravan is when the bikes are loaded. The Eriba hitch is typically now rated at 100kg and I'm guessing your car tow hitch is probably 75kg. You must not legally exceed the lower of theses two figures.
The way to measure the load is get the van sitting level, preferably on a hard surface. With either a nose weight gauge located into the hitch socket and sitting on a flat board or a stout 350/400mm ish length of timber instead resting on pair of bathroom scales. Raise the corner steadies slightly and the jockey wheel to see what the reading is. You may need to do some reloading by moving things from the front of the van towards the back, eg the contents of the dinette seating base cupboards onto the bathroom floor or possibly putting your awning onto the caravan floor between the heater and fridge. Beware of moving anything heavy to the very back of the van. It is better to have more stuff over the axle than a little stuff at the very rear of the van.

Colin