When there are two lanes in the direction of travel, it is not illegal to go by someone on the right. That is not considered "passing" when there are multiple lanes. However, I have to agree that showing a left turn signal but quickly switching back to a right turn is asking for trouble. Whether the person in the other lane is an idiot or was merely faked out by the signals, the chances of an unhappy ending for both of you are too great.
We've towed 5ws and driven long wheelbase motorhomes (with toad) and learned to be ultra-cautious in any right turn situation. Some of that knowledge was gained the old-fashioned way in the School of Hard Knocks. My basic technique is the fade to the left as much as practical in the situation, even while signalling right. It is also crucial to pull as far into the intersection as possible before beginning the turn. My co-pilot helps by watching the mirror on her side cause I also have to keep track of other traffic in front and left. Sometimes I have to intimidate traffic to get enough room to turn safely, pushing the nose out and waiting for others to get out of the way. Many drivers are courteous about your need for extra maneuvering space, but inevitably a few are not, or are at least reluctant to yield. I've received some rude gestures and comments in my time, but I can live with that. I try not to be arrogant in demanding a right-of-way, though.
Huey Pilot makes an excellent suggestion: avoid right turns in congested areas. In fact, avoid all congested areas whenever possible. I find it worthwhile to drive a few extra miles or take the long way around a block to avoid potential damage and the frustration of maneuvering in traffic. Most cities have truck route bypasses, so use them. And pay attention if a GPS routes you through a town - it is often avoidable. Most nav programs, even those touted for RVs, don't make routing decisions based on rig length. You can program to avoid bridges or tolls, but not to avoid right turns or narrow streets.