You say you'd put it on a pallet, do you have a forklift? If not then weight is going to be a big concern.
You could put a cassette toilet and sink on a pallet quite easily. The shower is an issue since you need an enclosure. If you can work with fibreglass you could build a shower enclosure.
However I think it would all just be easier as modular units. A standalone cassette toilet that could be placed anywhere. A small cabinet with a sink, water tank and a 12v pump.
If you wanted to do the shower inside you need a water supply, a water heater, the enclosure, plumbing and a waste tank. So I looked a little bit on Youtube. And I found a few videos of DIY showers.
Shower stall, pretty cheap.
Shower head, strange but also cheap.
Really neat shower head unit.
You still need warm/hot water though.
Mr Heater Base Camp Aqua Cube. Runs on propane, so caution must be used when using it in an enclosed space!