It looks like you might squeeze one in but not with wheels on. You should find out the dimensions of the one you want to make sure.It will be a pain even if you can fit one in there. You have the hoses, lance etc and you probably want to keep the back seats free of crap.
Or this...
You don’t have space in the back of that pick up for everything you Need for powerwashing
If you took the tank out you would have to carry a waterbut/wheelybin so it would still be tight. Just think about sticking 3 x 25lts of hypo in the back with a FSC, lances, buckets, shovel etc