As mentioned if you haven't already, pull the fuse and put your meter across to confirm its good. They aren't always obviously burnt and just doing a visual won't always tell you if its failed.
If you have your user manual for the furnace it shows the wiring diagrams in it. They are also available online at atwood mobile products: RV Furnaces - Atwood Mobile
The 7900-II and 8000-II series mount vertically, are wired identical and have the t-stat power tapped off ahead of the furnace breaker. There should be a red(+) and black (-) from the main fuse panel to the furnace. The red should be spliced near the furnace with a white tapped off and brought out somewhere near the front of the compartment with a second white coming from the control board. These will be spliced onto the t-stat wires. If this is your furnace I would meter the red/black supply at the furnace and make sure you have 12VDC there. If not it can really only be one of 4 things: bad fuse, bad connections, cut wire or a bad ground. If you have 12VDC at the furnace then I would jumper the two whites together at the furnace and your furnace fan should come on. If it does not then you likely have a bad ground if it runs then it's your t-stat or wiring between the furnace to the t-stat.
The 8500-IV and 8900-III series mount horizontally, are wired identical to each other and the t-stat power is on the secondary side of the switch/breaker on the furnace. If this is your furnace then start by re-setting the furnace breaker. If that doesn't fix it then check for power at the furnace terminal strip. Terminals 1&4 is the supply and should read 12VDC across, if ok then 2 is +12VDC to the t-stat and should read 12VDC across 2&4, if ok jumper 2&5 and your furnace fan should come on. If it does not then you likely have a bad ground and if it does then it's your t-stat or wiring from the furnace to the t-stat.