Rich,
Try to find at least 2-3 welding shops local and take the 5ver over for an estimate. It truly is the best and safest way to transport bike, unless they are inside the 5ver or on a pin box mounted bike rack.
If you don't mind me asking, how much were your bikes? Now call a dealer/ weld shop to ask how much to repair a 4" rv bumper. Add in the cost of the bike rack, and I am sure the replacement cost exceeds the cost for a custom hitch.
Don't take my post the wrong way, just trying to help! Good friends of ours bought a new to them tt, and I think it was on the way of their second trip another vehicle got them to pull over. Yes, the bumper broke, and had dragged the bikes, and they even lost one bike.
Our tt we just sold came with a bumper mounted receiver. There had been a repair made to the bumper due to it starting to tear. We never did use it so not sure if the tabs welded to the top of the bumper and I-beam frame would have been enough.
I was seriously looking at buying a front mount receiver for the truck, but our new tt has a factory frame mounted hitch we are hoping shows up in time for our next trip.
Good luck!