Harry,

It looks like the older horizontal propane tanks on RVs can't be upraded to
meet the current fill valve regulations:

http://www.damouth.com/RVStuff/OPD.shtml

Because of this they are exempted from the regulation but DO REQUIRE proper
labelling on the tank to notify the person filling the tank that it can be
overfilled.

Even if safely filled there are dangers:

http://www.rversonline.org/02Propane.html

When you purchase it, it's in a liquid state. Tank gets filled to 80%.
Allows for 20% expansion. When vaproized, it expands 270 to 1. Liquid
propane "boils" at -44F. If surrounding area is colder than that, it stays
in liquid form, and won't come out of tank. Not usually a problem w/propane.
However, as a rule fill your tank in the same general locality as you're
going to use it. Example to avoid: filling a tank in freezing temperatures
in the northern states, and driving to Arizona: When you filled the tank it
was filled to 80% capacity; but going to the heat of Arizona could cause the
propane tank to become "overfilled".

Propane expands 1.5% for every 10 degree increase in temperature. That's why
tanks are never filled beyond 80%. It's crucial to keep filled tanks so that
escape valve is on top so that any "overfilled" volume will be expelled as a
vapor, and not in its liquid form. This is why it's dangerous to paint
propane tanks a dark color to "match your rig" -- it will heat up to higher
temperatures and become "overfilled".

A 15 degree difference can make your tank overfilled if initially filled to
80%. A 50 degree difference can make it overfilled by 55% beyond 100%, that
is over most design safety margins!

I think the Canadian Government requires Canadian RVers to take a course in
propane handling and safety.