I don't think it's possible (at least from a practical standpoint) to manually extend the slide-out but you can manually retract it. Here's a link to the owner's manual, which you can download. Page 2-19 describes the emergency retraction process.
The fact that it won't work could be a simple repair or it could be significant. Page 2-19 also contains some basic troubleshooting steps. Basically, there are three major categories of issues, electrical, hydraulic and mechanical.
1. Other than fuses, electrical issues could have to do with the control unit or the wiring.
2. Hydraulic issues could have to do with the pump, fluid, the hoses, the hydraulic cylinders.
3. Mechanical issues would encompass problems with the guide rails, etc.
If you can describe what happens when you try to extend the slideout we might be able to help. Did the prior owner give you any info on the slideout issue?
Do your leveling jacks work OK? They operate off the same hydraulic pump as the slideout so, if they operate, you've eliminated it as a problem. Even if they're not working, can you hear the pump running? If not, the first thing I'd check is to see if the pump has a good ground connection. If you're lucky, that may fix everything. Make sure your fluid level is OK.
Winnebago has a host of online information relating to your MH, including wiring diagrams, plumbing diagrams, parts lists, etc. I suggest you download everything specific to your MH so you have them when you need them even if the internet is down.
(note that some links may be broken but Winnebago has been fixing them, so keep trying)
Also, here's a link to some info on HWH slideouts, however, they don't have information linked to a specific model Winnebago older than 2002, but you may find something helpful. Winnebago should be able to tell you which HWH system is installed in your MH.
Let us know the answers to my questions.
We love a new project. There's even a Winnieowner who works for HWH, the manufacturer of the slideout system who can help, but he and we will need more information.
One last thing. If you go to the User CP in the toolbar, you can set up a signature block similar to mine so we'll always know exactly what model Chieftain you have. Make sure you include your specific chassis since the Chieftain 33WB came with either a 6.8L V10 or a 7.5L V8 chassis.