Yes, it can replace your existing pull handle valve or go in front of it. Do a search for Baker RV dump Valve and it will come up.
Here is a discription of it:
Auto Drain provides a completely automatic 12 volt push button operation of the valves that dumps your holding tanks. Auto Drain installs on your current slide valve and provides consistent power to your valve which extends its life. Available for 1 1/2 inch and 3 inch slide valves. No plumbing modifications are required. Normal installations require 1 hour or less. Constructed of corrosion resistant stainless steel, aluminum and plastic. Models for Valterra and Bristol valves.
These are used extensivly of Class A's. Rereading your post, you will need two valves. One Black and for Gray water.
You can buy a Barker elecrtic RV dump valve for less that $120.00. It will need to wired up with a fuse or circuit breaker for a bit more cost.
You still will need to connect the dump hose on your knee or stooping over.