Use due diligence. Some of those entry-level diesel pushers are rated to tow less than 5,000 pounds.
When I was looking, a Thor Palazzo only had 4,000 difference between the GCWR and GVWR. Don't know the tow rating. Both Tiffin's entry level diesels are rated at or less than 5K towing. The Breeze is 4000 (28BR) and 4500 (32BR). The Open Road RED is 5K. I think the limiting factor on these chassis is the transmission--not sure though.
My 24k Ford chassis is rated to tow 6,000. Of course, an RV builder has to build a gas MH rated that high first. I haven't seen a gasser rated at 6K yet, but in theory we could.
My 24K Ford MH pulls the 4,500 lb Jeep just fine.
The engine will scream when you want speed. I just stick to 60-65 MPH and do just fine.
MPGs are about 7 with the Toad. I haven't taken long enough trips not towing to calculate MPG without the Toad.
So, watch out, some diesels will only tow the same as a gasser, but they will do it much more quietly.
Good luck in your search,
On edit: As usual, late on the trigger for me, but now that your mind is made up, some good info up there. Check the numbers on the chassis and see if the model you want can be optioned on the next size chassis up. Sometimes, that can be done.
