I replaced the thin steel square tube bumper on a new '84 26' Aljo 5th wheel trailer with a 4" x 1/4" wall square tubing so I could mount a receiver hitch for towing a 3500 lb boat behind the trailer. The Aljo's main frame rails ran all the way to the tube with two cross members just in front.
I cut the welds loose with a thin 3" dia radiac wheel with a 90 degree Dotco air tool.
I did the welding myself.
Sorry but I have no idea how many bucks it can cost in your area but a few phone calls around will tell the tale.
"good judgment comes from experience, and a lot of that comes from bad judgment" ............ Will Rogers
'03 2500 QC Dodge/Cummins HO 3.73 6 speed manual Jacobs Westach
'97 Park Avanue 28' 5er 11200 two slides