RV walls and siding are permanently attached
You will be adding lots of additional weight, that may overload the axles or frame
Springs or airlifts will raise the body keep it from sagging, but does not strengthen the chassis or axles, excess weight is a killer
Also you will have to seal every attaching hole made or water will get into the walls and weaken them
I would think very hard before doing this
If you really want this look
Shop for a house truck, built from the frame with house style framing and look
Look up "house trucks" on Google
Most are one off custom builds by the people that use them
Occasionally one comes up for sale
Many are works of art by master craftsman, absolutely beautiful