Firstly, as long as you keep the bags sealed and it’s dry in your boot, I don’t think you’ll have any problems with you powder.
As for getting accustomed to Huel, everyone’s body is different. I’ve seen some people on the forum say they jumped into 100% on day 1 and were fine. I would give yourself at least a month of easing in though, ideally 2. The reason being that in the first couple of months you may be more gassy and I am not exaggerating when I say your car will quickly become uninhabitable
I drive regularly abroad and use Huel. Usually pan continental for leisure and within the UK for work and leisure.
Huel powder is fine to be stored in the car. I am fortunate that my car has a saloon type boot and therefore my boot usually remains cool. However, you can place the bag infront of the aircon floor vent if the boot is warmer than 22C(room temp). If you dont wish to stop en route then you can pre mix the powder before you set off and sip slowly to delay toilet stops.
You may find RTDs useful as well. Using this method it is possible to limit the number of stops.
Using RTDs may be a better solution. On a road trip, it may not be as easy to prep the shakes. You could just crack a bottle open and drink without any fuss. And of course, I hope you enjoy your road trip!
I think I would do this, with a cooler in the back seat and some ice packs to keeps things chilly. You could always pop into supermarkets or convenience stores along the way to replace the ice packs as they warm up.
Thanks for the input folks. The problem I’ll have is that I have no rear seats, no air con, no roof and a very small boot (it’s a Caterham). My plan was to take the powder and top up my water bottles as I go. I’ve got plenty of time to test it out so I’ll see what happens to a packet left in the sun in a tin box.