AFAIK rv's overtake in the same manner one-way road or normal road. I'd say test it out with two stretches of highway, one with one-way and one without. Put same vehicles on both and let them start at the same time.
The only strange behavour I have noticed is that when you have two articulated vehicles behind each other (the first with slower speed than the second, so they are really close to each other), that non-articulated vehicles can't overtake them, because there is no gap between them. But that is the same for one-way or normal road. I don't think that vehicles see one-way roads as two seperate lanes.
allthough I might be wrong, maybe there is a pathfinder out there that does do that (in that case, which one)
Regarding the overlapping tiles, it could work (still danger of heavy glitching). But you would have to make tiles with different roadtypes on them (if this set is compatible with a couple of roadtypes) and you will need a heck of a lot of tiles for all the different ways that the overlapping is needed.
But if you want to stick to oneway roads, I recommend looking at this little GRF:
http://www.tt-forums.net/viewtopic.php?f=36&t=35787
Should be easy to alter the graphics of that and you could contact the original author to ask permission to use his code and graphics for a public release.
