There's a popular Youtube channel that has given instruction on configuring a manual switch to allow powerassist on both legs in a 50 amp RV with a Multiplus 12/3000/120 . https://www.youtube.com/watch?v=gbKBoIvXNPA
The wiring diagram is here https://allaboutrvsinfo.com/wp-content/uploads/2021/02/50-amp-rv-manual-switch-as-builts.pdf
The below comment from a viewer states that this is unsafe. Is it?
"Looking at your 50 amp solution. A 50 amp RV has two 120 volt AC 50 amp circuits.. The wiring is hot1-neutral-hot2 and a ground. Note that the voltage across the hot wires of hot1 and hot2 is 230 volts AC. The voltage across hot1 and neutral is 120 volts AC. The same is true of hot2 and neutral. However, the AC phase of hot1 and hot2 are opposite, the two cancel each other out on the neutral line. Your proposed switch is putting AC voltage across both hot1 and hot2 in the same phase. This is not safe and will cause the neutral to have a voltage and current when the loads across hot1 and hot 2 are different. Too much of a difference and the wares can heat up excessively. Too much heat and you can damage the wire insulation which could result in shorting and possible fire. I strongly suggest you have this switched wire configuration reviewed by a qualified electrical engineer familiars with 50 amp RVs, before promoting it as a single inverter 50 amp solution. As an Registered RV Tech and RV inspector I do not recommend wiring any RV in this manor."