Not recommended. There is the very real risk of damaging your slideout if a stabilizer jack slackened or a tire lost air on that side; causing the slideout to essentially be pushed up.
These were more common when slides first began to appear but the current electric and hydraulic slides can take the weight.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD