Floyd Mayweather -- Drops $300K On Land Yacht ... With 55 Inch TV, Bar & Massage Chairs!!

Floyd Mayweather dropped $300,000 on an insanely luxurious customized Mercedes party bus ... with a HUGE TV and BAR ... and had it shipped across the desert last night ... TMZ Sports has learned. The whip ... a souped up Mercedes-Benz Sprinter…