Hi.
We bought second hand cat 4 years ago and it was scared of everything! I think she had lots of emotional baggage and she found it difficult to deal with the hoover, tin foil, small children, plastic bags and people in general.
We often went into motorhome, which is situated in the back garden, and she would climb in and out freely to be with us. We eventually took the plunge and once she was in situ, we closed the doors and drove off. She hated it!! Wandered around, under the pedals, on the bed, in the cupboards etc. - not good.
Three years on we have a routine. She goes into her carrier, a slightly bigger canvas bag with a zip and mesh windows, we place it on the floor in between us in the cab and she sits quite happily on her cosy blanket. We make a point of putting the gas on whilst she is still in her carrier and only let her out when we are settled. At night, she often sleeps in her carrier quite happily.
We tend to use the cab doors to get in and out and she doesn't venture down that end of the van. We have also placed a piece of mesh inside one of the under seat cupboards (we can then let her in there and close the underseat door) which allows her to look out and be safe whilst experiencing some fresh air and a 'room with a view'. It is also convenient for us when we want to open the van doors and have a more relaxed situation. We have a litter tray in the shower room which she uses but I have put bluetac over the drain hole to stop any stray litter bunging up the water pipe. (use wood based litter to avoid scratching and both easily removed when showering)
We did buy a cat harness which is quite good, however, it needs to be fairly tight as cats wriggle. We let her have a walk when it is quiet. I have seen a motorhome with a cat flap in the door, the cat stuck within it's boundaries with no probs but I guess it depends on your own pet.
Good luck!!