Not that I have any plans to try this, but is it possible to put yourself on a diet on which you can survive that will eliminate the need to urinate and/or defecate?
(The things you think of on a long road trip!)
Sua
I think that you can cut back on your solid waste, but the only way to avoid urination is to stop drinking fluids or even eating and you will end up with either a kidney infection or dehydration.
Just stop to use the bathroom or buy some Depends.
Balance, Bob, balance. You drink enough excessive liquids to allow you to sweat, and sweat out the urea. It’s sheer genius, I tell you!!
Now, tell me why that won’t work. 
Sua
Try a low fiber diet. Very low fiber.
I suppose you could try to take your trip in a very warm vehicle to get the right amount of sweat. But then you will run into the dehydration problem.
Nope. Eventually, your body will want to get rid of its own waste products separate and apart from undigested food.
Dead red blood cells, for example, are responsible for the color of your, well, here.
You could turn yourself into a camel. IIRC, their “urine” consists of small dry pellets. Does that help? 
Bad idea. Your sweat glands can’t eliminate urea and conserve electrolytes the way your kidneys can, and there’s lots of other reasons why you can’t substitute your skin for your renals for filtering your plasma. For long car trips, just get a Foley catheter, insert, and keep on truckin!
As for poo, even a low residue diet won’t stop your colon from producing mucous, so you will still occasionally have to evacuate. It would be a lot less often tho
Qadgop, MD