In good conditions, my GMC Duramax can go 500 miles on a tank. In perfect, 600 miles. On straight #2 Diesel. Diesels are easy to convert to CNG.. Add the tank, control system (regulated by boost pressure) and a nozzle in the intake to fumigate the intake with CNG. The kit is very affordable. I considered doing the conversion but the tall pole in the tent was a place to fill the CNG. I have natural gas in my house, but the pump cost $15K last I checked and had to be rebuilt every 40 hours of use. The only other places to fill the CNG around me were municipal filling stations.
How big a tank?
You can buy a lot of diesel for $15k.
The original manufacture of the Phill home CNG unit went out of business, mostly due to their quality problems and poor reliability many years ago. The rights to the name and product were purchased by BRC, a commercial CNG refueling manufacture.
The housing may look similar, but it is a far better unit and should be closer to $5,000 installed unless you have other onsite issues.
http://www.brcfuelmaker.it/phill-domestico-prodotto-brc-fuel-maker.aspx
Also, depending on where you live, more CNG commercial stations are opening up.
Natural Gas Fueling Station Locations
http://www.afdc.energy.gov/fuels/natural_gas_locations.html
In some locations, lease option of the compressor is also available.
http://www.northwest.williams.com/Files/Northwest/AGL_LDC_NGV_Mtg_Fuelmaker_Presentation_2-29-12.pdf