The 50 amp units will cover a 30 amp shore power as well. You wind up using only half of a 50 amp unit when connected to 30 amp shore power. But, if you buy a 30 amp unit, you won't be able to use it on a 50 amp feed. So, buy the 50 amp version.
I use the
50 Amp SurgeGuard hardwired version but another popular unit is the
Progressive Industries 50 AMP hardwired unit. Either one works great.
Generally you just remove the shore power cord from the transfer switch and cut 2' off the end. Then splice the surge protector into the cord and reinsert to your transfer switch. When you trade in your coach you just remove the surge protrector and the next owner gets a cord that's 2' shorter.
By the way - I wouldn't "consider" getting a surge protector - I'd definitely go get one. They also protect against low voltage brownouts. The consequences of messing up your electrical system due to low voltage, surges, or lightning spikes are VERY expensive. I personally consider these things mandatory.