Twine / Glind water pressure

won't be good enough if you want high pressure

Define "wash".
OK for rinsing off. But couldn't use to blast mud off for example.

As said above, you can certainly rinse the vehicle with the shower, but unless you get a higher pressure wash-down pump, which will probably blast the dirt off you, the normal shower pressure probably won't be enough to do the troopy. The flow rate of a wash-down pump will probably be a fair bit higher than a shower pump, so if you are drawing water from a limited supply (I use an onboard bladder)(on board the car, that is) a washdown pump is likely to empty the tank pretty quickly,whether showering or washing the vehicle. Check out a boating supplies shop like Whitworths if you want to source a washdown pump.