Some trailers have enclosed tanks and valves, other have it all exposed. Mine is enclosed and heated. If enclosed there'll be a remote control system. Either a cable or electric drive system. I have used Valterra valves in the past on my 5th and a a TT I recently rebuilt. The standard Valterra valves will work in all assemblies I've seen. The cable control has a clam shell that mounts to one side the valve and provides an attachment point for the remote control cable. You might, but probably not need to replace the cable assembly as well. Yes I'd replace both the 3" and 1 1/2" valves at the same time and get it over with. Sometimes its easier to get the required clearance between parts to loosen both the bolts on valve assemblies. Probably depends on your tank arrangement. Hint, you might want to position your trailer so that the drain end of the tanks is uphill to reduce any remaining residue liquid from running out as you open those valve bolts.
Fred & Denise
Santa Fe, NM
2007 Excel Classic 30RSO 2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB