steering in a 1985 Toyata HiAce auto campervan

Hi, I am new to this . I recently purchased a 1985 Toyota with only 129,000.00 kms [ true kms] on the clock.
As it only has a 1.8 or 2 litre motor with a trakka fit out it is very, very slow. I have found it hard to steer sometimes non power has a steering box. By hard to steer I mean sometimes tends to wander a little bit particularly on the highway with trucks coming the opposite direction. Hard to stay within lane across the where lines are quite tight. As stated if trucks are approaching or even other vehicles can be hard to drive straight.I am presently having the steering box overhauled but the guy rebuilding the steering box said it isn`t in bad shape. Next option I guess is to look at the shockies. The van has new tyres light truck travelled $5000 kms. [ home brand from Bob Jane].As the van is so old I don`t want to keep spending heaps of money on it. The steering box guy suggested I ask the mechanic to check the rear leaf spring bushes and the centre bolt in the leaf springs. So annoying when no one can diagnose the problem. I am a non mechanical girl. Thanks.