Les,
Good choice going with Steve.
We have the 55 litre tank in the wardrobe in the rear corner of the van (it only takes up half the space and we didn't use the wardrobe anyhows.
You could go for a tank under the seat - this depends on how much space you have under there - it does vary - Autosleepers have a tight space.
Filler point is in the rear corner panel side low down - on the outside.
Leisure tank is under the fridge on the underside of the van (20 litres I think we have). (Same stuff - Propane - in both tanks)
When you fill up at a pump - it automatically overfills into the leisure tank (meaning you always have a full leisure tank - very handy) so you never worry about how full it is (should be good for several weeks of camping - we have heating, fridge and hob/grill running from it).
How do you know how full the tanks are?
Don't worry about the leisure one - there will be plenty.......
You get a little LED block with 5 LEDs by the steering wheel.
When you fill up you get 5 LEDS
We get 4 at about 60 miles - down to three at 100 miles ish then you don't worry too much about the other two - just expect to fill up between 150 and 200 miles (20 ish mpg at the mo).
This will be different with a bigger tank.
When you get it done - buy the two continental adapters from steve if you plan to go abroad.
Any other questions - come back to me (But not about the technical details

........)
Mikey