the most suited camper would be the Burstner delfin performance based on a renault 2.5 either 120bhp or 140bhp. this is a tagaxel overbed garage 2/4 berth and are a real barhin at the moment seen plenty 2005 to 2007 models for around the 39-42k mark.
not sure on the rear garage capacity try emailing burstner in Germany unless "lowdhams" know the capacity bu as kev said 2 bikes will weight at leat 300kg with fuel and thats a lot of weight on the rear axel [ the bustner has a payload of 1000kg]
how about a small twin bike trailer?
The only other good manufacturer is Dethleffs which we have and are excellent but again th max single rear axel is 150kg in the garage due to rear overhang and thats on a 3900kg chassy with a payload of 700kg.
As with everything there are short comings