Look at the power plug for your RV, Then read parts 1 and 2 of this post:
If the plug has 3 prongs, 2 flat one round or "U" shaped: That is a 30 amp cord. you can use a 30 amp supressor on a 30 amp rig, it will work just fine.
If the plug has 4 prongs, 3, flat and one round or U shaped: Then 50 amp. Note in this case you should pick up an adapter so you can plug into a 30 amp outlet if you need to, you might also wish to visit http://www.psrv.net
Using the 30-50 and 50-30 adapters you can use a 50 amp surge/power proteector on any rig, 30 or 50 amp, in fact it will work BETTER on a 30 amp rig, much better in fact since it has several times the spike supression ability of a 30 amp unit. This also means if you were to Upgrade to a 50 amp rig during the life of the supressor.. No need to buy a new one.