Before you rush out and spend $130'ish on a new pump, check the shower head and pull out the water restricter (if there is one). Otherwise- Shurflo Flojet, model 03626149, is what we have for our external draw pump. Flow rate is 13 l/pm and gives us reasonably good pressure at the shower head in the van when drawing from a jerrycan on the ground. Has a current draw of 11amp at max pressure of 40psi, so a 5 minute shower uses about 1 A/H of your battery storage (if my maths is correct).
Cheers