I too have these strange routings. I do know that i can set priority for fastest or shortest routes and it is supposed to automatically use my preference. Sometimes i think it forgets my preference. So now i ask it to compare routes before telling it to "go.". Unfortunately, the shortest route very often does take you off the highway, or put you through the center of a stop light ridden business route through town versus staying on the interstate by using a by-pass. I would think that predictive traffic or real traffic, or traffic everywhere, the latter two being extra cost items, would adjust for this, but they do not.
Now i could understand this for a shortest route that takes a lot more time than the fastest, but this is not the case. Something is clearly wrong with the logic used. On my non rv garmin i could set assumptions for city driving (15-20), county roads (45-55), and interstate (60), you are stuck with rand's default. Rand's adjust trip length feature in theory should fix this, but it does it to entire route not by road type. It does work if all you do is 90-100% interstate for the trip..
Regarding three right turns versus one left. Detailed manual discusses this, saying it recognizes that some rvs are more difficult to turn left, but rands solution is pick a different rv type in your rv settings. You must guess how much smaller of an rv you need to get access to normal left turns. This should be an avoid in city. Or always avoid feature, but it would be better as a knowledgeable of the route logic feature. I don't think anyone has that detailed road knowledge.