Nah they dont have things like that in the manuals.
Half the stuff is only made for a couple of years before the design changes.
The rest of the stuff is standard across several manufacturers.
Try the local dealer or camping shop with a spares department.
Draining the heater is easy. Unscrew the white plug/valve on the outside of the van.
Then with the battery/pump switched off open all the taps to let air into the pipes.
When we are finished on a campsite i remove the screw and leave it out all the way home. The water should find its way out over the course of the journey.
I leave the taps open also.
Just remember to close them when refilling the next time you use the van.
Check if it a Sprite Leisure model or a Swift built one as it was about that time that Swift took over the manufacture of the Sprite brand. If it is Swift made then the parts will be common with most Swift group models of that era but if Sprite Leisure made you might have a few probs as I don't think Swift took on much in the way of parts when the purchased the name.
------------- 'A sure cure for sea-sickness is to sit under a tree'
You could contact the secretary of the Sterling Owners Club; it was the Sprite Owners Club before the Sprite name went, but it is now back.
He may be able to put you in touch with someone who has a manual. http://www.sterlingcaravanowners.co.uk/contact%20us.html