an RTV is for Road Taxed Vehicles, IE ones that have not been modded. So short of a set of knobly tyres there's very little you can do. (but this depends on how strict your club rules are)
To make the most of your trial get some practice in... the last thing you want is to go out on all stages at the first gate. this'll just dampen your day! Learn your 'steering lock' and the 'width of your vehicle', go find a carpark or better still a lump field and setout a couple of canes just wide enough to drive in between, then start to drive through them without stopping, reversing or hitting them from a whole bunch of different angles and in different Low gears.
Most club rules will allow one 'shunt' per section for a disco, so know when to use it and simply try not to.
You will be suprised how capable the disco is, let it handle the ground while you concentrate on pointing it in the right direction getting your line and getting it clean between the markers!
Most of all enjoy it..!