Some manufacturers will work with you and install or change/modify whatever your heart desires when you buy new, others will outright refuse any request for customization.
When you buy used, look at the bones (solid structure, no water issues, leaky roof, etc.) and make all the changes and improvements you want. The caveat is weight and space. You probably don't want to mess with the configuration since this would involve changes to the plumbing and electrical system, even simple add ons like a fireplace can overload the electrical system in some trailers. Keep in mind that a trailer is somewhat sensitive to weight imbalances, front to back and left to right and building materials are not only chosen based on cost but also weight.
Many RV owners make some changes after they buy but big remodeling jobs are usually not worth the trouble..
2013 RAM 3500 Longhorn Dually, Andersen Ultimate 5th Wheel Hitch,2014 Trilogy 3650 RL, 8000# axles, disk brakes, I.S., 960 Watt Solar