I would also echo Rick's comments. When I started my mobile/portable adventures I had two new certificates generated. When operating mobile or portable I put the grid and state I am operating from in the comments section my logging program as a reminder of where I was when the contact was made. Later I generate the adif and upload to Lotw using the station location I had setup. The only problem I have encountered is that I cannot figure out how to automatically download those contacts back into my log after they are processed by Lotw, but since I don't make more that a couple of dozen contacts I usually go and look at them on the Lotw page and manually update the log.
I have tried various methods of doing the same with eQSL and have found it to be very cumberson, so I gave I up and just use KC3EF without any other designator. I like the fact that I can delete a Lotw station location when I am done with it, but eQSL ones live forever. I find that some people use both, but a lot of non-North American ops us eQSL.
Dick - KC3EF