I'm thinking you're planning for way too much amperage. Shoot, the alternator on my old 1/2ton puts out about 60A tops and the alternator on my 3/4 dodge diesel is rated in the 80A range. What alternator are you using? Are you aware that you might be using close to it's entire rated capacity to charge your RV battery?
Your RV battery must be totally dead at that point! I would be looking at carrying a small generator or increasing the size of my RV battery bank (i.e. more batteries hooked in parallel) before I asked my vehicle to put out that much charge current. I've got a dual battery/isolator setup for snowplowing in the winter, and I don't think it even draws as much current as you're planning for....and it will burn up alternators if I'm not careful.
For battery longevity, it is suggested to not take deep cycles down below 85% charge or so, and I believe a 20A rated circuit will be plenty for charging in this scenario. Your battery will not last long if you're hammering them flat and them hitting them with 30A-60A charge current. I typically charge my starting and RV batteries with a 1.5Amp "Battery Minder" charger around the place because I don't kill them, but keep them topped off. Let me ask you, are you familiar with battery chargers? What models do you use, and what amperages are they?
I hope you find this helpful. From your first post, It seemed you were familiar with bulk/float/smart charging. Maybe you're planning for some non-typical scenario other than normal camping functions and I'm not seeing the whole picture.... I'm just telling you what I understand, which isn't much sometimes......
I would think an inexpensive relay (i.e. Bosch) would be a great way to go for your switch idea (like they use for wiring up off-road lights and such). You would then run a second wire to the cab to monitor the voltage. (This is why I said you will be basically running the same #of wires through the cab whether you switch the charge wire directly or use a relay to switch it). The Bosch type relays are made in a wide range of amperages even up to 70A like you're talking about. However, in the larger amperages, a starter solonoid is probably a cheaper way to go, but they are typically not rated for continuous use like you found out. Relays & solonoids basically accomplish the exact same thing.