I prefer road bikes. But: as you've toured extensively on your current bike, which more than proves its worth, I'd start with the question of "what, if anything, didn't work on your previous tours?"
If it worked well, and it's in good mechanical shape, then by all means use it.
If there are some minor points (e.g. saddle, hands felt sore, gearing wasn't low enough) then you can address the specific issues.
In your situation, I'd only consider a whole new bike if:
- you didn't find the MTB comfortable, after trying some basic changes
- you want drop bars instead of flat / trekking bars
- you feel like the geometry is WAY off (e.g. the bike isn't stable when loaded)
- you've got major concerns about the integrity of the frame