Hi D
I have a Powrtouch on board mover fitted, as I have to get the caravan up some ramps to get over the kerb and into our garden. I fitted it myself in an afternoon and am very pleased with the result. It will certainly go in a straight line as you have a motor on each wheel. Each one is individually controlled, so it will also piroutte on the spot with the wheels going in opposite directions at the same time.
I fitted one to my father-in-laws 'van last year as his drive is hard to get into and he can't stop singing its' praises.
The advantage of on board movers is of course that they go with you, so you can use them on site. Great if the grass is wet and slippery, and good for raising one side up a ramp for levelling. The only caveats are that you may need a larger battery, or a second one in reserve, and it will of course add to the weight of the van. So you have to be careful if you are already at the loading limit of the 'van or the car.
If you're interested then go to:
www.powrwheel.com