I have a 7x5 trailer, which is used for bikes, 3 in fact, I have know idea how you could fit 5 bikes on a 7x5 trailer, unless of coarse 3 are very small, I struggle with 3, 1 TTR250, 1 XT225, 1 CRF230, our 7x5 weighs around 300 to 350kg and with 3 bikes on and gear is about 800kg, how many of the bikes are full size(ie) Moto X, Enduro type bikes?
A 7x5 trailer will go most anywhere in the bush, most fire trails can handle a fire truck, so car and 4WD should be fine, our camper is 9x5, so 7x5 for us is a piece of cake, turning around and very steep or steep and rutted tracks are the major concern.
When towing you have to make compromises and plan ahead a bit more.
Baz.