We've got fleet trucks locally that switched over and are saving big bucks yearly, you can order commercial trucks pre-converted from Ford and GM. From what I've read..the engine requires hardened valves which many engines already have.
Has there been any talk of converting MH coaches to LPG or CNG in the forums or better yet..anyone who has done it?
CNG Prices

A Dual Fuel LPG conversion might eventually pay you back for the conversion cost however I would
be curious where you would put the LPG tank of a large enough size to have a decent range. LPG is not always easy to find when you need it thus the
need for retaining the ability to burn gas.
Because of the size and weight of the tanks needed for CNG, the very limited range and virtually no availability of refueling stations
it is not an option at all for an RV its an urban fleet solution at this point. Either way
I think it would cost you money not save it!

I've done a lot of research on the Compressed Natural Gas. I'm fortunate to live in Central Oklahoma with a compressed natural gas station on the south side of town. It could work for me but here are the problems I found out:

1. Size of tanks---Most CNG tanks have about 6 inches of fiberglass surrounding the actual inner tank. Therefore, what appears to be about a 40 gal tank (comparing to an LPG tank) is only a 6-8 gallon tank. That put your range around 100 miles. It takes a HUGE tank to have any kind of a range... The other thing is it takes a long time to fill the tank...probably 10-15 minutes to get 75% or overnite to get more....

2. To have any kind of range, you need a DUAL FUEL setup that runs on gasoline and CNG. MOST factory conversions are CNG only....most aftermarket are dual fuel.

3. The conversions via certified converters run around $15,000. One reason they are so high is, as I was told by a converting firm, is each conversion firm has to be permitted and certified to convert each specific engine size. I think this permit was around $10,000 per engine series. In other words, you have to have a permit for each individual size of GM or Ford engine. The converter I talked to was only permitted to convert Ford 4.6 liter engines or GM 6.6 liter engines. He couldn't afford to buy permits for other engines. He had these permits because they were the most common commercial vehicle engines in fleets.

4. One other thing, is the CNG runs a high pressure....3000 psi? I think? Anyway, you want a certified converter with GOOD AMERICAN EQUIPMENT. In other countries, they use cheap CNG equipment and you can read stories of the vehicles blowing up and very few pieces of car etc....are ever found....

Hope that helps....

Excellent info guys but I also found a couple of sites that show some potential.
cleanenergyfuels
This site shows virtually every type of vehicle on the road that have been converted but as K&E pointed out, there are some big issues especially for MH owner's.
rasoenterprises
This company does converstions.

One other thing I remember now...

I looked into the CNG home refueling station that is hooked up to your home natural gas.

ONE CAVEAT...the pump was around $3000. It was only good to pump 3000 gallons of CNG before the built-in computer shut it off so you would have to replace it.....in other words, the cost of the at-home refill system was $1 per gallon on top of the fuel cost....