I cannot fully answer your question, but it seems that New Hampshire simply uses the ICC guidelines for building codes, including for "Plumbing, Mechanical and Fuel Gas" which seems to be the key area here.
Not being an ICC member, I can't access the documents, and neither the New Hampshire Statutes website nor the State Building Codes website reprints these documents for public viewing. This may be to encourage the use of licensed individuals for work subject to code, but IMO as "ignorance of the law is no excuse" in most jurisdictions you should be able to educate yourself on the building code free of charge, the same as for any other section of law. I would call your county clerk and ask how to get ahold of a copy of the codes. It may cost you a little money for their time and paper, but if they can help you here it probably won't be as expensive as an ICC membership for yourself.
If the government won't help (excuse my cynicism but I'd put money on various offices passing the buck), if you need chapter and verse, just get it from both the propane guy and the gas company. They're telling you two very different things, and they can't both be right. Ask both companies for a hard copy of the section of the building codebook, with title and year of that document, detailing the relevant codes which back up their understanding of how it should be done. Both are working for you, and one has shafted you; my money for which one is with the one who can't quote the most recent official codebook recognized by the State of New Hampshire.
The most important code sections to be aware of when installing outdoor receptacles, are 314.15, 314.23, and 406.8(B)(1).
First off, the box should keep water out.
National Electrical Code 2008
ARTICLE 314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures
314.15 Damp or Wet Locations. In damp or wet locations, boxes, conduit bodies, and fittings shall be placed or equipped so as to
prevent moisture from entering or accumulating within the box, conduit
body, or fitting. Boxes, conduit bodies, and fittings installed in wet
locations shall be listed for use in wet locations.
Secondly, the cover should also keep water out even when things are connected.
ARTICLE 406 Receptacles, Cord Connectors, and Attachment Plugs (Caps)
406.8 Receptacles in Damp or Wet Locations.
(B) Wet Locations.
(1) 15- and 20-Ampere Receptacles in a Wet Location. 15- and 20-ampere, 125- and 250-volt receptacles installed in a wet location
shall have an enclosure that is weatherproof whether or not the
attachment plug cap is inserted. All 15- and 20-ampere, 125- and
250-volt nonlocking receptacles shall be listed weather-resistant
type.
Lastly, you'll want to make sure the thing isn't laying in the mud. Depending on the Authority Having Jurisdiction (AHJ), you might get away with attaching the box directly to the tree, or supporting it with the conduit used to supply it. The proper way to do it, however, would be according to 314.23 (B). Use a stake.
314.23 Supports.
(B) Structural Mounting. An enclosure supported from a structural member of a building or from grade shall be rigidly supported either
directly or by using a metal, polymeric, or wood brace.
(2) Braces. Metal braces shall be protected against corrosion and formed from metal that is not less than 0.51 mm (0.020 in.) thick
uncoated. Wood braces shall have a cross section not less than nominal
25 mm × 50 mm (1 in. × 2 in.). Wood braces in wet locations shall be
treated for the conditions. Polymeric braces shall be identified as
being suitable for the use.
Common sense says you should install the box up out of puddles, and if you get snow in your area above the normal snow level. There are no code sections that deal with the height above grade, so use your best judgement. And never forget the most important code section of all...
110.12 Mechanical Execution of Work. Electrical equipment shall be installed in a neat and workmanlike manner.
Best Answer
In this state the power pole is regulated by the requirements on the utility company, but must be inspected and approved by the AHJ. The most important thing to remember about any temporary power set up is that all power must be GFCI protected. There are some questions about breakers that serve feeders other than standard power outlets being GFCI protected. I have found, in this area, it varies between each jurisdiction.
Here is a standard Temporary Power Pole assembly issued by the utility company that always seems to work if you follow it to the letter.
A standard temp pole only has two 20A 120V circuits GFCI protected, and one standard 20A 2P 240V circuit connected to a suitable receptacle, but non GFCI protected. Like I said some jurisdiction say its ok and some make you remove and blank out the breaker. Sometimes Jurisdictions will allow you to install what is called a spider box with extra receptacle outlets on it, but they also must meet NEC and jurisdictional codes. If you take a good look at note 4 on the image you will see that most temp power poles are 40 or 60 amp feeders. I generally will go with a 60A feeder.
In conclusion what I am saying is that there are pre qualified types of power pole directions that cover all of the NEC requirements without having to go through the entire code to get it right, and is perfect for a DIY party to construct. If you take a good look at the image I posted. How many code sections would you have to address just to build a temporary power and hope you got it right? As for myself I would just go with what they say will work and move on.
Hope this helps and good luck.