SChema

<?xml version="1.0" encoding="UTF-8"?>
<model package="campermanagement" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM">
    <object class="cmCamper" table="extra_campers" extends="xPDOSimpleObject">
        <field key="brand" dbtype="int" precision="10" phptype="integer" attributes="unsigned" null="false" default="" />
        <field key="type" dbtype="varchar" precision="255" phptype="string" null="false" default="" />
        <field key="plate" dbtype="varchar" precision="255" phptype="string" default="" />
        <field key="car" dbtype="varchar" precision="255" phptype="string" null="false" default="" />
        <field key="engine" dbtype="varchar" precision="255" phptype="string" null="false" default="" />
        <field key="manufactured" dbtype="int" precision="10" phptype="string" null="false" default="" />
        <field key="beds" dbtype="int" precision="10" phptype="integer" attributes="unsigned" null="false" default="" />
        <field key="weight" dbtype="int" precision="10" phptype="integer" attributes="unsigned" null="false" default="" />
        <field key="mileage" dbtype="int" precision="10" phptype="integer" attributes="unsigned" null="false" default="" />
        <field key="periodiccheck" dbtype="int" precision="10" phptype="integer" attributes="unsigned" null="false" default="" />
        <field key="remarks" dbtype="varchar" precision="255" phptype="string" null="false" default="" />

        <field key="price" dbtype="int" precision="25" phptype="integer" null="false" default="0" />
        <field key="status" dbtype="int" precision="10" phptype="integer" null="false" default="0" />
        <field key="keynr" dbtype="int" precision="10" phptype="integer" null="false" default="0" />

        <aggregate alias="Brand" class="cmBrand" local="brand" foreign="id" cardinality="one" owner="foreign" />
        <aggregate alias="CamperOptions" class="cmCamperOptions" local="id" foreign="camper" cardinality="many" owner="local" />
    </object>

    <object class="cmCamperOptions" table="extra_campers_optionslink" extends="xPDOSimpleObject">
        <field key="camper" dbtype="int" precision="10" attributes="unsigned" phptype="integer" null="false" default="0" index="index" />
        <field key="option" dbtype="int" precision="10" attributes="unsigned" phptype="integer" null="false" default="0" index="index" />

        <aggregate alias="Campers" class="cmCamper" local="camper" foreign="id" cardinality="many" owner="foreign" />
        <aggregate alias="Options" class="cmOption" local="owner" foreign="id" cardinality="one" owner="foreign" />
    </object>

    <object class="cmOption" table="extra_campers_options" extends="xPDOSimpleObject">
        <field key="name" dbtype="varchar" precision="255" phptype="string" null="false" default="" index="index" />

        <aggregate alias="CamperOptions" class="cmCamperOptions" local="id" foreign="option" cardinality="many" owner="local" />
    </object>

    <object class="cmBrand" table="extra_campers_brands" extends="xPDOSimpleObject">
        <field key="name" dbtype="varchar" precision="255" phptype="string" null="false" default="" index="index" />

        <aggregate alias="Campers" class="cmCamper" local="id" foreign="brand" cardinality="many" owner="foreign" />
    </object>
</model>

Snippet

$campers = $modx->getCollection('cmCamper');
echo '<pre>';
foreach ($campers as $cmp) {
    $stuff = $cmp->toArray();
    $stuff['brand'] = $cmp->getOne('Brand')->get('name');
    $stuff['opts'] = $cmp->getMany('CamperOptions');
    foreach ($stuff['opts'] as $opt) {
        $stuff['options'] = $opt->toArray();
    }
    print_r($stuff);
}
echo '</pre>';