The slides in existing RV's are made by humans that have skills that most people can learn. They are not something magical but, like any project, they do require a plan before your should attempt to undertake such a project. That is why I suggested contacting a salvage yard and see if you can find a similar coach with the items needed to make it work. You may even be able to contact the manufacturer of your coach and get the plans on how your existing slide is constructed. Keep in mind that most manufacturers have an eye on the bottom line, which doesn't always translate to the best quality. If you get the plans to make a slide, you may find ways to improve the installation.
Any welding shop can do the welding that is needed and a lot of them have portable rigs to come to you to do the project.
Personally this does not sound like too big of a project, especially if you can get a set of plans to work from. From what I have seen, the slide is built independent of the RV on the production line and then is installed later. They also seem to be built on a raised platform so that they are basically in line with where they will be installed. Working this way, you can do a lot of work on it before you have to open up the wall on your coach. Have fun and don't start this project until you have enough information to finish it.
