Yes it does seem the values can vary between labs, we had a post recently, where I think one lab was reporting about 5.5 and the other about 4.5 so 20% difference.
I believe you can order PSA tests online, they send you a test tube and a syringe and you send it back by post. You could do this whilst you were travelling, as long as you had some sort of postal address, campsite whatever. Of course you would still have the non comparable results so it would only tell you anything useful if the change was quite large perhaps going up to 3 or more.
If the last test was April the next one must be due in July, I would try and get one done just before you set off on your trip, even if it is a few weeks early, and then maybe bring your road trip back near home in six months for another test, if you really feel it is that important to keep on top of the tests.