Mine is an 2011 so it has the drop down frame in the front. As Carl said that makes it much easier to do the 2 front tanks. If yours is not a drop down frame I would cut the coroplast covering from side to side behind the front valves, then remove screws towards the front to access the valves. If you are mechanically inclined, you will figure out how to do it. Its pretty simple.
As for the galley tank, that's a little more difficult. Find where the drain line comes through the covering and cut around it in a 2 foot or so square. Only cut 3 sides leaving one side uncut (the front side) so you can fold it down, do your work and fold it back up and tape it shut.
2018 Chevy 3500 LTZ Dually Diesel 4x4 CCLB
2011 Montana 3455 SA. 6 point level up. Disc brakes. Curt Q24 Hitch
Progressive EMS. Valterra tank valves. Sailun G637 tires. ARP fridge control. All led lighting. Mor Ryde IS