Nice upgrades to the running gear, and you won't be likely to find those on a new unit. Regarding the interior, it can get expensive quickly depending on what you want to do.
Cabinets are made by the manufacturer using plans designed for each floorplan, so they are essentially custom cabinets. You're not gonna be able to get replacements from Home Depot. However, you can update the looks of the cabinets by painting them or by using a refacing company to put new faces on the frames and new doors. A lot depends on what you have now and what you want it to look like. New floor is easy to do. Appliances may or may not be easy depending on what you want and whether they fit in the existing space.
If you are talking about paying somebody to gut the interior and put in new everything, it will be $$$.
Google
RV Remodelers, there are a bunch of folks that do this work including
Lazy Days down by you in FL. They have a custom cabinet shop and can do whatever you like...