It is best to try it in AC mode first since this doesn't require the burner. Set the thermostat to maximum cold. After you turn it on, wait an hour and then open the lower outside access door and feel the shroud around the boiler (looks like a piece of stovepipe). It should be warm to the touch if the element is working; the tube coming out the top of the boiler shroud should be getting hot by this time. If no heat, then either no AC to the refrigerator or a bad heating element.
In summer weather, it takes a while to cool down. After 2 hours, the freezer compartment should be getting pretty cool. It will take up to 3 or more hours before the refrigerator part is noticeably cooling. By overnight, the refrigerator should be getting down to temperature.
If the boiler is getting hot and the refrigerator is not cooling by this time, the cooling system is shot and it is time for a new refrigerator or a rebuild.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10