While perusing the RV catalog(s) for a new toilet I kept seeing ads for anti-siphon valves.
On the advice of the RV Service Manager here in town I eliminated the valve on our Seeland Travelor 511 about eight years ago when I replaced the seals.
His reasoning? Spring loaded actuator lever to initiate a flush or fill. The spring shuts off the valve. There is no ball float to stick. If the spring loaded actuator is working correctly--no water thru, no reservoir tank above the bowl to drain. The main supply pump should be off when RV not occupied and constant running would indicate a problem.
It worked for me.