We tried a few lashups with tarpaulin with very little success.
However well tucked in the bikes were, they got mucky.
So bad that we decided not to bother, and left them out in the open. At least the rain rinsed them then.
In the end the solution was to buy a motorhome with a 'garage' - which is just a poncey name for a 'boot'.
In the boot, the bikes travel clean, dry and warm. They don't make the van longer and they are not visible to miscreants.
Hopefully you find bike covers that do the job and are not a huge faff to fit and remove.