Am I right that this is a very short length of coax in a vehicle, and
the VSWR will be low due to the ATU at the feedpoint, and that the ATU
ground / coax braid will be connected to the vehicle metal framework?
So why use thick coax? Why the choke?

If you need a choke, you could get some cores that are a neat fit over
RG58, at 10MHz appropriate type 31 cores has a Z of 115/inch. That
doesn't address the issue of whether the power / control connection
needs protection.

Actually, I am not sure why you are doing this anyway, is it just in
case or is there a demonstrated need?

Owen


Well, to be honest, I was primarily planning on using the choke for a
base station application.... will have some sort of balanced dipole or
loop fed by an SG-237 tuner. I wanted the choke for the coax feeding the
tuner.

On the RV, I have a 23' whip on the back (ladder) which I had planned
on feeding with another SG-237. ( Currently it is fed with a home-brew
base loading coil assy, on which I must manually change frequency taps.)
In either case, I thought it might be beneficial to put a common mode
choke on the RG8X feeding that antenna, too, since I do have RF causing
various issues inside the coach while I am transmitting.

On your RV grounding question, yes, the antenna feedpoint and all
related ground areas are very well bonded to the main vehicle chassis
with rather large tinned braid, (2" wide).

Ed