most of the info needed comes with the unit,unless you buy secondhand,
choose a position for it and mark where the inlet and exhaust pipes go.drill a 3 or 4mm hole through the floor and push a bit of wire through the hole,then look underneath to see if its all clear for the two pipes to come through.
if clear,drill 2 holes at40mm[ or more if you want ]
connect the metal flexi exhaust pipe to the exhaust pipe,the unpainted one, with a jubilee clip,if you don't have a metal pipe for the inlet,use a 30mm waste pipe for caravans,also clipped on.
shove pipes through holes and screw down heater through side brackets,get under van and fix the exhaust pipe just under the side skirt with a metal clip,making sure its facing to the rear of the van and is bent down a bit so it doesnt fill up with rain ! clip the inlet pipe facing back at least a couple of feet from the heater exhaust
take the black and red wires,through a 5 amp fuse,to the leisure battery or a fuse board for 12volt supply.
the other wire is for the control box which is mounted somewhere sensible about halfway up the wall same as a house thermostat
the gas fitting on the back of the heater is 8mm,so decide what gas pipe you'll cut to branch off from,turn gas off,and use a 8mm 'T' joint[ £6 in the caravan shop, £1.50 from screwfix]to connect pipe from heater.use an inline gas tap if you like,and i use a sealing compound like calorseal
the large hole on the right of the heater front is the inlet and is fine as it is,the ducting is clipped to the left hand and taken to the black adjustable outlet
for those who don't know,these heaters take air in through the inlet pipe,mix it with gas and burn it in a sealed chamber,the exhaust gas then comes out under the van through the heaters exhaust pipe-very safe. at the same time a 12v fan sucks air from the van living area,through the hole in the front of the heater,and blows it past the heated chamber,which heats the air,and out through the ducting iback into the van.