Re: 528E/5 as an RV Dinghy?
It's a manual transmission right? (I'm guessing that's what you meant by 528e/5 ?) With an automatic you can't tow it with the rear wheels on the ground (unless you disconnect stuff) but with a manual there's no problem.
As for attaching the tow bar, the front bumper shocks slide into and fasten to the frame rails. If you take off the bumper and shocks, maybe you could bolt on a pair of suitable brackets in their place? I think the shocks were held on with only 3x M6 bolts each, so you'd have to modify it for something better than that at least.
I towed my 320i by attaching the towbar to the bumper, with the bolts going through the steel bracket behind the bumper as well as the aluminum. Of course the e28 is a heavier car, and I don't know how sturdy the bumper shocks really are.