Check out harbor freight or northern tool. I'm thinking they have the multi fuel generators. Great choice btw!

 

another opion is the gov liquidations site they sell surpluss military gear. 18KW multi fuel gen in A or B condition is worth the 500.00 to get..

 

You tube has some vids on drilling and tapping the carb to plumb in a gas line and convert anything to tri-fuel. Obviously some convert easier than others.

 

I'm not sure this meets the "multi function" definition but I'm giving serious thought to this one Yeti 1250 Solar Generator Kit because it can be used indoors.

 

The Survival Podcast just did a two part series on generators a few weeks ago. Definitely worth listening to. It covers everything from the best types of generators are to how to hook up a transfer switch...

Steven Harris on Everything Generators Part 1 of 2 | The Survival Podcast

Steven Harris on Everything Generators Part 2 of 2 | The Survival Podcast

 

If your gen has the required wattage, just convert it to Tri-fuel. I did mine for $187 and it works great. You just have to order the proper conversion kit. I went slow and it took me about an hour, following the directions.
A and C Propane Natural Gas Conversion Kits