I'd have thought though you really only want the welder for the break down emergency. Your never going to set this up to undertake a project weld job surely? on that basis carrying a full sized welder in the car seems silly? also if you get the welding job done and your battery gets you home then its done its job. If it last another year then its even better.

When i did the Madigan TK in 2012 the hill with us had a shock collapse and break in the middle of nowhere. We has a 24v mig with us an it did the job with ease and ran off one of my batteries and a second from another car. I replaced that battery only last week. I put in on charge yesterday and the smart charger said it was fully charged..

I think for touring a set of Jumper leads will do the job you need and get you home. I like your idea but it seems like overkill.