There is some difference between the small bar antenna and the patch antenna of the Sleek cradle. I would expect more feedback from the small bar antenna, than the Sleek cradle. You should be able to shield the Sleek with some foil. You may have to ground the foil--but try aluminum foil, and a small alligator clip to ground if necessary. Then you can make an aluminum shield if necessary. At this frequency this should work.
Moving the antenna 4 feet up, may also help, plus it will give you a higher point--and slightly longer range. The marine antenna has identical radials as the truck antenna.
I am also doing some experimenting with the mag mount antenna and a 18" Pizza pan (which has been coated with primer and enamel, plus adhered to the RV roof with 5200. This same "pizza ground" can be used for my 2 meter and 220 and 440 mhz antennas. I'll see if there is feed back from this, but I suspect that there will not be any.
As far as the distance from the patch antenna--I have not pushed to see what the limit is. This was just a trial run, and it was working fine at 4 feet. I will be putting the i phone on the dash of the RV with a bean bag mout exactly the same as my Garmin GpS mount. The Auto Nav Router will also be close to the small bar antenna. I will be experimenting with the antenna on an extendable pole before drilling holes in the RV and putting a ball mount on the side of the RV with the truck mount extension to get the antenna up as high as possible (same height as the satellite dome) . (Although the RV skin is mostly fiberglass, there is a steel "Cage" all around the driver's area--back about 8 feet from the front. The Wilson amp, and router are both set up so that they can be transfered from RV to Car to Boat.
Another option, I have is an aluminum "brush" pole which goes from 8 feet to 16 feet, along with an extension for the antenna cable (there will be some loss with the extension). I can move this from place to place, or even raise it up higher.
Toyman, I am surprised that you are getting feed back in the van. I didn't in the Honda pilot. I had the antenna back about 5 feet from the driver's seat. You might try the shielding in the house/ or van.
One final thought, with these UHF frequencies, you can make a parabolic or even a "can" type of directional antenna. When reasonably stationary, the radiating element is short enough that you can make a screen reflector behind the antenna--which might help both with feedback and increase of gain. Again, I'll experiment with this (probably use some aluminum sheeting to make my reflector.