I had that concern when I first started journalling, and invented a secret script (neography). I stuck to it a week or two before I realised nobody really even cares about most of the stuff I write about, so why make the effort. Mind you, I'm young and don't live with a partner, so I don't have to worry about them finding my journal.
I might be embarking on a similar trip next year—same route. I'm not familiar with the American climate; What would be the ideal time of year to do such a journey? February/March to July/August?