Spencer Pratt is digging in over the backlash surrounding his mayoral campaign ad ... insisting he never lied about living in an Airstream trailer after being called out for actually staying at the ritzy Hotel Bel-Air. The candidate joined "TMZ Live" Wednesday after we broke the story ... he's been staying at maybe the swankiest hotel in L.A. ... this after calling out the luxury homes where incumbent Mayor Karen Bass and challenger Nithya Raman live. But Spencer says people are missing the