Ok I have a ridiculous set of circumstances that I am unsure how to tackle and could use some professional advice. 

The scenario: Making a 3 tier fondant cake for my son's 5th birthday. Party is Sunday, 4 hours away from my kitchen. We are leaving Friday afternoon. 

My concerns: Making the cake and traveling VS. Making the cake once we arrive. My main concern is that the location I'll be at on Friday (were I to wait and make the cake after arriving)  is my grandmother's house, who refuses to turn on her air conditioning, and it will be humid and in the 70s. I think I can figure out a storage situation were I to make the cake at home and transport the tiers, then stacking the day of the party. Working with a cake/fondant in a hot and humid house scares me. I don't know which is worse, if either. 

Any input is appreciated! 

My opinion is to make it as much as possible at home.   Not sure what size(s) but transporting could be a problem.  You certainly could cover the tiers w/fondant and maybe put some/most of the decorations on.....then all you would have to do at the last minutes is put the tiers together and border them.  Just be sure to take along *everything!* you could possibly need in an emergency tool kit.  I agree working in an unfimilar kitchen in hot humid weather would only add more tension to an already sticky situation. 

The sizes are 6, 8, and 10 inch tiers at 4 inches high. I have a minivan so my options for transport are much more convenient in that regard. What would you recommend for temp regulation? Should I refrigerate them once I get there? Keep them room temp in coolers? Thank you so much for your input! 

Definately frig them on arrival.  When sizes are that close in size (6,8,10) there is much more chance of the cake falling over in transport.  You might put two of the tiers together but I would not put all 3 stacked - just too much of a chance of problems even in a van w/lots of room.   If you do choose to put 2 tiers together, put them on a base board that is much larger (like 14-15") - that helps spread the weight & force of turns and makes a firmer base.  Then put a long dowel down the middle of the 2 tiers and make sure it sinks into that bottom board which should be at least 1/2" to1" thick.  Once you are at venue, you can remove it. 

My plan was to transport unstacked and then stack on the day of.