At 12VDC and 30 amps, the losses are significant with only 3 or 4 feet of 10 gauge wire, equal to about one third of a volt. If it takes 20 feet of wire to reach the trailer connector at the back of the truck, plus another 10 feet from the hitch to the battery in the camper, the loss could be a show stopper.
For a 12V battery to accept a charge with a resting (fully charged) voltage of 12.4 volts, it must be in a circuit with a voltage of at least 13.4 volts (This is a critical number.). Assuming a voltage of 14.5 for the charging system under light load, it is easy to see the probability that your camper battery will not be charged more than about half capacity with the arrangement proposed. Practical reports will verify these estimates.
Your camper can work by doing this, unless it is planned to run a microwave, frige, computer, TV set, and a big sound system (exaggeration herein to make the point).
John, keep this in mind:
those losses you speak of diminish as the battery charges. This is because the current flow drops off.
Please look up "IR" losses and see what happens when the current flow drops to one amp and lower.
Those losses will simply disappear! Eventuall the battery will see the full voltage from the alternator....well almost, they never go aways completely.
10gauge solid copper wire has just under an ohm per thousand feet! thats not much loss at 10 amps and of no consequence at 1 amp.
IR drop calcs using 40 feet of 10 gauge solid copper wire:
@ 30 amps the loss is 1.2 volt
@ 10 amps the loss is .4 volt
@ 5 amps the loss is .2 volt
@ 1 amp the loss is .04 volt (float charging is usually less than an amp)
His setup will work just fine, and will fully charge a battery.
He only has one risk issue: popping his fuse during the initial charge, bulk, phase. That why the camper manfs use a 30 amp thermal breaker. Even that fuse popping is a pretty low risk.
