They are cheaper to buy, but the quality and durability of everything makes them expensive. Things wear out or break fast. Everything is sealed with shitty sealant that is prone to cracking and leaking, and needs to be resealed every 3-5 years. If there is a leak, the moisture stays trapped and starts rotting out everything. The roofs are covered with a thin membrane that breaks down from the elements and UV, and can get holes ripped in it from twigs. Fuel to travel is expensive. RV parks are somewhat expensive. Poor insulation and inefficient HVAC. Heating and cooking (and water heating and sometimes refrigeration) with small propane tanks is inconvenient and somewhat expensive. Some full time RVers find it costs more than before they started doing RV full time.

There are good quality RVs that mitigate some of these issues (Airstreams and Class A), but they cost as much as houses in some areas in the US.