Monaco custom built their own chassis called the Roadmaster chassis. It is very heavy duty and was based on an early Chrysler military vehicle design. If it's not a tag axle coach, it will have 8 air bags and 8 shocks. The Executive is the second from the top of the Monaco lineup. The Signature is the top model.
The Cummins ISM is not the engine that is having issues with the #6 intake valve, that is the 650hp ISX. The ISM 450 engine was an expensive option on a 1999 40' coach. The newer ISM engines after 2003 can have EGR (exhaust gas recirculation) issues, This early engine should be great.
I would not call the 40' model clumsy, but it will have a wider turning radius than a 36' coach. If you find a similar model with a tag axle it will turn as tight as a 36' coach because the wheelbase is shorter than a non tag 40' coach and the tag axle lifts.
Most of these big coaches (and many smaller ones) get 7-8mpg depending on how they are driven.
I'm not sure I would purchase this coach if you only intend to have it for 1 year. Most buyers are looking for at least one slide out. There are a few that do not want slides because of the complexity, but they are difficult to find when selling. Also, a 1999 coach will probably need repairs and updating and you can easily put a lot more money into it. As an example, batteries can be $1000, and tires are around $4000. It will help if you can do the repairs yourself.
Monaco does not list the 1999 model brochure, but you can download the 2000 which is very similar.
Hope this helps.