I'm afraid that 65psi would put me in the left-hand ditch as soon as I tried to make a left-hand turn..
"It is very near impossible to have vehicle with 5000#+ on the rear axle have the rear end step out on drive pavement and normal speeds"
That makes sense, and it doesn't really feel like that would happen, so that is confusing. So thinking out loud: what would cause oversteer without a tendency for the rear-end to come around... Hummmm. Guess I will lay awake tonight thinking about that...!
No tail wagging. No complaints on how it tows and I don't have any indication of too much tongue weight: thinking of my 21'boat tongue weight on my Tahoe. The RoadMaster tow bar doesn't place any tongue weight, to speak of, on the towing vehicle, just pulling weight.
In my original message, I did mention that the van is a 210, if that changes any of your ideas.
The van has factory Aluminum wheels.
Another thing that is confusing me is that my net searches bring many videos that discuss anti-sway bars. It seems that I am hearing that a front sway bar can cause oversteer ??? Now, I have never heard discussions on sway bars causing over or under steer. When I was into autocrossing sport cars we heard one used a front sway bar to keep the rear wheels from lifting off the ground. I have seen cars on the course lift one of the rear wheel off the ground in tight turns. These were cars not set up for autocrossing and that's why they had the problem. Those of us that set our cars up for that type of event, always installed a front anti-sway bar. For over/understeer I would adjust tire air pressure. I can't find any link that will specifically answer the question of keeping the weight on the opposite end of the installed sway bar.
There is something really odd going on, for sure. Higher pressure should not but you into a severe over turning situation. I don't really think it is classic oversteer because that is lack of traction on the rear end.
The information that you saw about swaybars is correct even if it is counter intuitive. They work backwards from how you would assume. Increasing the size or adding a swaybar to one end improves the traction of the other end. In your autocross example that meant getting both tires on road with more weight on them in the rear and the front bar did that. That means that adding a rear bar would improve understeer by improving the front traction. But that isn't what you have.
The only thing that I can imagine that would do what you say is happening is if the front wheels are self steering and that can only happen if something is not right in the geometry.
If a vehicle is aligned with negative caster it will self steer as soon as it is off center with no input from the driver. The old manual steering, heavy, rear drive cars were aligned that way to give them "light effort" steering. They were also all over the road trying to keep them straight.
Another is if the toe is set out instead of in. As soon as the outboard tire passes the straight ahead position in it's transition to turned in the turn rate will increase noticeably. If there is any slack in the front end parts or steering gear that extra turn in can be quite large and unnerving. Road race cars use towed out as it gives good turn in response and holds corners better but I think the also use large positive caster with it. Toe out is bad on the street vehicles though as it makes them less directionally stable and makes turn in feel odd. Toed out cars are often called twitchy.
Very loose front end parts could also probably cause what you feel. Once you start to turn all the slack goes out. Sometimes there is a point where you actually can feel it in a car that is tight in the front parts, from what I saw on my 96 Buick Roadmaster wagon that has similar front steering as the vans. In a turn as I turned the steering into the corner I would feel a point that the steering got easier to turn, just a slight oddity. I had no idea why and all the parts were new including a high end modification steering gear with better feel. That car needed more caster but the adjustments couldn't get there, so I cut apart the upper control arms and rewelded them back together to give the car more caster. With the increased caster the odd spot in turns went away completely. That makes me believe that as the wheels turn the caster changes depending on the design of the system. At some point the caster was going from positive to negative in the steering travel and that was what I was feeling. By adding more positive caster initially it made that point either go away completely or move to a spot that it didn't matter like only at full turn.
What you are describing sounds a lot like what I saw with the Buick, but on steroids. I think to get as much extra turning as you describe something may have to be loose and moving. Control arm bushings or bolts maybe. Upper control arm pivot bolts and cams may be broken or loose. Idler arms being loose is very common in the vans because many techs and owners miss the very well hidden upper grease fitting. There is an outside chance it could be rear steering also if it has a bad spring or shackle bushing but that rarely happens.
You have a 210 with aluminum wheels and are towing so you have the trifecta of stuff to be careful with.
210s are heavy to start out with because of the extended/larger coach section. The are something like 16" longer than stock so center of gravity moves toward the rear. Depending on how they are optioned, we have seen 210s mentioned that had barely 500# of load capacity before getting to max GVWR or overloading the rear axle if weight is placed too far back. Add a few hundred pounds of tongue weight hanging even further to the rear and you move the center of gravity more to the rear and add more than the tongue weight to the rear axle load. The front gets lighter with the more rear weight is hanging off the rear. This is why it is very important to get the axle weights, or even better the corner weights, on a 210, especially if it is towing.
The aluminum wheels are very likely to be zero or -6mm offset based on the ones folks have checked. Our 07 had different aluminum ones than yours and they were -6. Stock GM steel wheels that came on the vans are +28mm so with the aluminum wheels the tires move out nearly 1 3/8" at each wheel. This can mess up steering geometry and does put more load on all the wheel bearings, particularly in the rear if you have the standard non towing package semi floating design rear axle.
The rear axle max loading spec is basically the tire loading max spec of 6080# so it really means 3040# per side to protect the tires. The Roadtreks tend to have 2-300# more on the left rear than the right so you can see that you can have an overloaded tire even if you are within the axle limit. This is why corner weights get important.
Unless you are a relatively hardcore DIY mechanic, you do probably need to find a shop that is familiar with the light trucks. Pickups had nearly identical systems so an offroad shop might be a choice, or the previously mentioned trades/delivery type truck shops. They very possibly will be able to spot what is wrong, as what you see if very unusual for the Chevies, I think, as I can't ever recall your kind of issue being mentioned before.
