I too raced motorcycles and wanted to wash them off before putting in the trailer. At first I added a water facet to the underside of camper and used the freshwater from the trailer tank to a pressure washer.
Pressure washer was 115v so if premitive camping (like at most races) the 12v camper pump feeds the 115v pressure washer (so I needed to run generator) and then it used valuable water from camper fresh water tank - had to manage it well if I wanted to save enough to take showers!
I too was trying to workout a solution to add water capacity to camper - but cost and location was making that difficult!
Then I found the best solution yet:
As suggested previously - get a seperate tank that can live in the truck bed - it is 12v so it can run from truck power and doesnt use up our camper water. This way we still had water to shower and water to clean bikes and best of all it was a cheap solution! You also have the option to use the seperate tank for other jobs around the house/farm!
Got mine at Tractor Supply.
Fimco Spot Sprayer, 15 gal. - Tractor Supply Online Store
Mine is just like this one: Pressure not that high but gets bikes clean prior to loading. And for the price - you can't beat it. Under 100 bucks, you will pay more than twice that for a tank to fit under trailer and then you have to fabricate mounts etc.