At this point in our build, we do not have the entire layout perfectly planned and pinned down, but we were pretty sure of how the propane tank was going to end up so that work could be done before the interior layout was settled.
Out of the box
We selected a Manchester 6814 7.9 gallon tank that is 10″ in diameter and 23″ long.
Deciding where to install
We thought that this was a good compromise between capacity and clearance. The tank is designed to be mounted under the floor of RVs.
We looked at putting the tank in front of the axle:

But that would require re-routing the parking brake cables — not an impossible task — but it would be a lot of work.
As more time was spent under the van we realized it could go behind the axle, and in front of the spare tire:

The spare tire is a little close to the propane tank valve, but I think I can fix that later. Plus, I liked that all the valves and connections would face towards the rear, away from flying gravel picked up by the front wheels.
Propane tank install step-by-step
The Manchester Tank 6814 comes with some small, thick brackets, but since we are mounting to the floor of the van, we wanted to spread the mounting area out and give the tank more fore-aft stability. We got some 3/16″ steel plate, did some trimming:

Aimed the press with the home-made bender attachment at it:

Finished brackets:

Test fit of the bracket to the tank:

Here is the tank installed behind the axle, in front of the spare tire, using the new brackets:

We used one of the original propane tank brackets to modify the spare tire bracket:

This bracket holds the spare tire towards the back of the van, making sure it does not bump into the tank. The black is some 3M undercoating to cover up the raw edges and prevent rust.
We put some 1/4 inch straps between the floor ribs to give a strong mounting:

We used heavy, countersunk flat head bolts to keep the floor smooth. To make sure that there was a good bond with no rattles or squeaks, we put a layer of 3M outdoor mounting tape under the straps, above the van floor. We also put a layer of this tape on the top of the mounting brackets between them and the bottom of the van floor.