If I remember correctly the RV65 is a 75 ohm system between the external antenna and the amplifier. The older system is highly likely to be 50 ohm, so you WILL need to run new cable. I suspect the reason Wilson switched to 75 ohm on the RV 65 and a few other RV systems is that low loss 75 ohm cable is much less expensive and easier to handle than low loss 50 ohm cable.
The wilson comment on stationary use only is for regulatory compliance. Mobile systems are limited to 50db gain, stationary systems can have more gain. If you want to use it with a fixed antenna you will also need to get a 75 ohm antenna.
The antenna from inside the trailer to the amp is still a 50 ohm system.