Replace refurbish Thetford Waste valve

[Quince]

Looks like I need to replace my Thetford Waste Valve on my 68' overlander. No Idea if it is original or not. Do I need to come in from below to remove that? Looks like I need to take off the underbelly? Can someone confirn the right wat to attack this thing? Like to replace the waste level sensors while Im in there. is that doable? or is that something that is only addressed from the top side?
Suspect I also need to reemove the toilet and get a new seal. We have been getting some smells from inside...

Any advice about replacement parts and procedures for this work would be greatly appreciated.

not gonna be fun, but looking forward to getting back to non-leaky valve.

Thanks!
Q