Lenders take a lot of different things into consideration. There’s the basics like income and DTI, but there is also your unsecured debt to income ratio (the ratio of total unsecured debt vs your annual income), net disposable income (amount of money you have left each month after all reported debts are paid. Time at your job, time within your job industry, length of time at your address, whether you rent or own your home. Also, lenders typically prefer your DTI be below 40% WITH new debts figured in. So if OP is already at 40%, he will be above that amount once the RV loan is figured in.

ETA: mortgage payments are viewed the same as any other installment payment. Student loans can sometimes be an exception, but are typically figured at 1% of the balance for monthly payment. The only debt that typically holds almost zero weight is medical debt.

You’re going to pay penalties for your account going negative no matter what. When an account goes negative, it’s not just collections you need to worry about. It’s your Qualifile score that’s gonna tank. When you try to open an account anywhere, your most likely account is going to be whatever “second chance” account that institution offers. These accounts are often restricted. Sometimes you can’t have debit cards, only ATM cards. They’re often accounts with heavy monthly maintenance fees. And if you’re thinking you can just close the account and immediately open a new one to avoid the reporting, sorry to say many institutions now do repeated Qualifile checks after you open an account. So you might slip through initially, but after that one month mark when they review and potentially pull it again, you’re at risk of that account being shut down too. Trying to just dodge the issue isn’t really the best way to go about this.