You probably know that motorhoming is about making compromises. Some cyclists compromise on living accommodation so they can have a garage which should be more secure for bikes. Others buy Panel Van Conversions, some of which are designed to house bikes inside. Depending on your alarm fitted, you might find you can extend protection to incorporate the cycles. Bringing the bikes into the van is doable - I've done it - but at the risk of damaging the bikes and the van's furniture. You won't be able to drive with the bikes inside - I can't see how they will be secured, so it will be a case of removing the bikes each time - with the risk somebody will be watching.
We have in the past carried bikes inside the van [not carbon fibre ones]. We stripped the wheels off and rested them upside down, covered in thick blankets to keep oil off the furnishings and jammed them in with old cushions. - you might be able to put up the bed and lie them on that. They will be visible to passers by.
Finally, just buy a pair of cheap bikes for when in the van.
You may want to ask your insurer as they will have experience of bike theft risks.