Mona, your 1998 is just one year older than my 1999 Classic. The refrigerators may be the same. On my Dometic, you set the switch to GAS, then push in a red button on the same panel. Hold it in until the needle in the indicator gauge moves and comes into the green section. That means the fridge is lit, and you can stop holding in the red button.
The gas setting seems to be the most efficient of the three, cools better and faster than either of the electric settings. In fact, I start to freeze things in the refrigerator section when the temp control is set only on 2.5.
Since different Hi-Lo models seem to use different refrigerators, your best bet is to get the proper manual for the exact model you have and read up on their operating instructions and if that is not enough, the trouble shooting section. You should be able to get a copy of the manual from the manufacturer's web site.
1999 22L Classic
2008 Toyota 4Runner V6